Why is the text in these table column cells not centered?

Question

I have the following table in my document, and it seems to be OK except that the topmost columns' text is not centered. I can't figure out why not. I could manually adjust the multicolumn m sizes to get there, but that does not seem like the right solution.

I have a similar table in my document and the centering works fine. This is all within a 2 column IEEE-type document. I thought maybe my table was too wide and something weird was happening - but making the table less wide does not solve the problem (although it does make the text off center in different ways). My entire table is centered on the page; I tried taking that out and leaving it left justified and that had no effect.

The TeX that generates this table is:

\begin{table*}
   \centering
   \caption{Results on Xilinx Virtex 5 Devices}
   \label{tab:XilinxResults}
   \begin{tabular}{|m{3cm}|m{1.50cm}|m{1.50cm}|m{1.30cm}|m{1.30cm}|m{1.30cm}|m{1.30cm}|m{1.30cm}|m{1.30cm}|}
      \cline{2-9}
      \multicolumn{1}{>{\centering}m{3.0cm}|}{ } &  % blank box
      \multicolumn{2}{>{\centering}m{3.0cm}|}{\bfseries Clock Frequency (MHz)} &
      \multicolumn{2}{>{\centering}m{2.6cm}|}{\bfseries Throughput (Mbit/s)} &
      \multicolumn{2}{>{\centering}m{2.6cm}|}{\bfseries Area (CLB Slices)} &
      \multicolumn{2}{>{\centering}m{2.6cm}|}{\bfseries Throughput / Area} \tabularnewline \hline
      \multicolumn{1}{|>{\centering}m{3cm}|}{\bfseries Architecture} &
      \multicolumn{1}{>{\centering}m{1.50cm}|}{\bfseries 512-bit} &
      \multicolumn{1}{>{\centering}m{1.50cm}|}{\bfseries 256-bit} &
      \multicolumn{1}{>{\centering}m{1.30cm}|}{\bfseries 512-bit} &
      \multicolumn{1}{>{\centering}m{1.30cm}|}{\bfseries 256-bit} &
      \multicolumn{1}{>{\centering}m{1.30cm}|}{\bfseries 512-bit} &
      \multicolumn{1}{>{\centering}m{1.30cm}|}{\bfseries 256-bit} &
      \multicolumn{1}{>{\centering}m{1.30cm}|}{\bfseries 512-bit} &
      \multicolumn{1}{>{\centering}m{1.30cm}|}{\bfseries 256-bit}
      \tabularnewline\hline\hline

      \bfseries BMW x1 &
      \raggedleft 4.89 & \raggedleft 8.14 & \raggedleft 5004  & \raggedleft 4168 &
      \raggedleft 12039 & \raggedleft 6164 & \raggedleft 0.42 & \raggedleft 0.68 \tabularnewline \hline
      \bfseries BMW x1-PPL18 &
      \raggedleft 46.51 & \raggedleft 70.97 & \raggedleft 47628  & \raggedleft 36335 &
      \raggedleft 24564 & \raggedleft 11610 & \raggedleft 1.84 & \raggedleft 2.96 \tabularnewline \hline
      \bfseries BMW /32(h) &                                                                                    
      \raggedleft 44.27 & \raggedleft 69.41 & \raggedleft 1416  & \raggedleft 1110 &                            
      \raggedleft 4001 & \raggedleft 2211 & \raggedleft 0.35 & \raggedleft 0.50 \tabularnewline \hline          
      \bfseries CubeHash x1 &                                                                                   
      \raggedleft 152.70 & \raggedleft 151.93 & \raggedleft 2443  & \raggedleft 2430 &                          
      \raggedleft 745 & \raggedleft 672 & \raggedleft 3.28 & \raggedleft 3.62 \tabularnewline \hline            
      \bfseries CubeHash x1-PAR5 &                                                                              
      \raggedleft 151.26 & \raggedleft 151.56 & \raggedleft 12101 & \raggedleft 12125 &                         
      \raggedleft 3742 & \raggedleft 3490 & \raggedleft 3.23 & \raggedleft 3.47 \tabularnewline \hline          
      \bfseries CubeHash x1-PAR17 &                                                                             
      \raggedleft N/A & \raggedleft 150.49 & \raggedleft N/A & \raggedleft 40933 &                              
      \raggedleft N/A & \raggedleft 11360 & \raggedleft N/A & \raggedleft 1.91 \tabularnewline \hline           
      \bfseries CubeHash x1-PAR33 &                                                                             
      \raggedleft 150.65 & \raggedleft N/A & \raggedleft 79542 & \raggedleft N/A &                              
      \raggedleft 24570 & \raggedleft N/A & \raggedleft 3.24 & \raggedleft N/A \tabularnewline \hline           
      \bfseries CubeHash /2(h) &                                                                                
      \raggedleft 173.52 & \raggedleft 184.71 & \raggedleft 1388 & \raggedleft 1477 &                           
      \raggedleft 733 & \raggedleft 624 & \raggedleft 1.89 & \raggedleft 2.37 \tabularnewline \hline            


   \end{tabular}                                                                                                
\end{table*}                

Answer 1

When using \multicolumn{1}, you don't need to respecify the column width via m{<len>}, since it requires a cumbersome interface when trying to centre the column entry. Rather just use \multicolumn{1}{c}{...}.

Here is an updated version of your code snippet:

\documentclass{article}
%\usepackage[landscape]{geometry}% http://ctan.org/pkg/geometry
\usepackage{array}% http://ctan.org/pkg/array
\newcommand{\stackcell}[2][c]{%
  \begin{tabular}{@{}#1@{}}
  #2
  \end{tabular}%
}
\begin{document}
\begin{table*}
   \centering
   \caption{Results on Xilinx Virtex 5 Devices}
   \label{tab:XilinxResults}
   \begin{tabular}{|>{\bfseries\arraybackslash}l|*{8}{>{\raggedleft\arraybackslash}m{13mm}|}}
      \cline{2-9}
      \multicolumn{1}{c|}{} &  % blank box
      \multicolumn{2}{c|}{\bfseries \stackcell{Clock Frequency\\(MHz)}} &
      \multicolumn{2}{c|}{\bfseries \stackcell{Throughput\\(Mbit/s)}} &
      \multicolumn{2}{c|}{\bfseries \stackcell{Area\\(CLB Slices)}} &
      \multicolumn{2}{c|}{\bfseries \stackcell{Throughput /\\Area}} \\ \hline
      \multicolumn{1}{|c|}{\bfseries Architecture} &
      \multicolumn{1}{c|}{\bfseries 512-bit} &
      \multicolumn{1}{c|}{\bfseries 256-bit} &
      \multicolumn{1}{c|}{\bfseries 512-bit} &
      \multicolumn{1}{c|}{\bfseries 256-bit} &
      \multicolumn{1}{c|}{\bfseries 512-bit} &
      \multicolumn{1}{c|}{\bfseries 256-bit} &
      \multicolumn{1}{c|}{\bfseries 512-bit} &
      \multicolumn{1}{c|}{\bfseries 256-bit}
      \tabularnewline\hline\hline

      BMW x1 & 4.89 & 8.14 & 5004  & 4168 & 12039 & 6164 & 0.42 & 0.68 \\ \hline
      BMW x1-PPL18 & 46.51 & 70.97 & 47628 & 36335 & 24564 & 11610 & 1.84 & 2.96 \\ \hline
      BMW /32(h) & 44.27 & 69.41 & 1416 & 1110 & 4001 & 2211 & 0.35 & 0.50 \\ \hline
      CubeHash x1 & 152.70 & 151.93 & 2443 & 2430 & 745 & 672 & 3.28 & 3.62 \\ \hline
      CubeHash x1-PAR5 & 151.26 & 151.56 & 12101 & 12125 & 3742 & 3490 & 3.23 & 3.47 \\ \hline
      CubeHash x1-PAR17 & N/A & 150.49 & N/A & 40933 & N/A & 11360 & N/A & 1.91 \\ \hline
      CubeHash x1-PAR33 & 150.65 & N/A & 79542 & N/A & 24570 & N/A & 3.24 & N/A \\ \hline
      CubeHash /2(h) & 173.52 & 184.71 & 1388 & 1477 & 733 & 624 & 1.89 & 2.37 \\ \hline
   \end{tabular}
\end{table*}
\end{document}

Hopefully this improves some of the formatting and readability of code. Some improvements include:

• Using a repetition column specifier: *{<num>}{<col spec>} which repeats <col spec> a total of <num> times;
• Inserting \raggedleft at the "column specification" level to avoid cluttering the tabular contents with \raggedleft;
• Using \arraybackslash within >{<stuff>} in the column specifier, which restores the traditional use of \\ as \tabularnewline;
• Added \stackcell[<col spec>]{<stuff>} which adds a stackable tabular within a cell (although you may not need/require this).

Finally, as a recommendation for professional-looking tables, consider using the booktabs package. The horizontal alignment/structure of a table lends itself to avoid using vertical rules, something promoted by/when using booktabs.

(geometry was loaded in landscape mode to make the table fit on the page when compiling the MWE.)