Two tabulars not coming out same width despite every column width specified with p{}

Question

I have two tabulars in a single table environment. They should be aligned but they are not (see photo), despite me having specified all the column widths and given them the exact same setup. Can anyone help with this mystery? Removing the multicolumns in the first table doesn't help. Thank you.

(The contents of the multicolumn cells should also be centered.)

Font changes removed from the MWE.

\usepackage{booktabs}

\newcommand\tss{\textsuperscript}

% ...

\begin{table}[!htbp]
\centering 
    
\begin{tabular}{p{15mm}>{\centering}p{15mm}|>{\centering}p{15mm}|>{\centering}p{15mm}p{1mm}>{\centering}p{15mm}|>{\centering\arraybackslash}p{15mm}}

\cmidrule[.8pt](l{3pt}r{3pt}){2-4} \cmidrule[.8pt](l{3pt}r{3pt}){6-7} 

Pong & a\tss{1} & \multicolumn{2}{p{30mm}}{a\tss{3\,5}} &  & aːt\tss{7} & at\tss{9} \\

\cmidrule(l{3pt}r{3pt}){2-4} \cmidrule(l{3pt}r{3pt}){6-7} 

& a\tss{2} & \multicolumn{2}{p{30mm}}{a\tss{4\,6}} &  & aːt\tss{8} & at\tss{10} \\

\cmidrule[.8pt](l{3pt}r{3pt}){2-4} \cmidrule[.8pt](l{3pt}r{3pt}){6-7}  
     
\end{tabular}
    
\begin{tabular}{p{15mm}>{\centering}p{15mm}|>{\centering}p{15mm}|>{\centering}p{15mm}p{1mm}>{\centering}p{15mm}|>{\centering\arraybackslash}p{15mm}}
     
\cmidrule[.8pt](l{3pt}r{3pt}){2-4} \cmidrule[.8pt](l{3pt}r{3pt}){6-7} 
    
Viet. & a & á & ả &  & át & ắt \\

\cmidrule(l{3pt}r{3pt}){2-4} \cmidrule(l{3pt}r{3pt}){6-7} 

& à & ạ & ã &  & ạt & ặt \\

\cmidrule[.8pt](l{3pt}r{3pt}){2-4} \cmidrule[.8pt](l{3pt}r{3pt}){6-7} 
     
\end{tabular}

\end{table} 

Answer 1

\multicolumn{2}{p{30mm}} isn't as wide as the two columns p{15mm}|p{15mm}, as between the two columns there is also the space of two \tabcolsep plus, if you loaded array, the space of one \arrayrulewidth. So for your \multicolumn use

• \multicolumn{2}{p{\dimexpr30mm+2\tabcolsep}}{...} if the array package isn't loaded

• \multicolumn{2}{p{\dimexpr30mm+2\tabcolsep+\arrayrulewidth}}{...} if array is loaded.