hyphenation rule not acted upon [closed]

In this example, why is the hyphenation rule not used?

\documentclass{article}
\usepackage{tabu}
\hyphenation{abc-def-hij-klm-nop-qrs-tuv-wxyz}
\begin{document}

\begin{center}
\begin{tabu} {|X|X|}
abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz &
abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz
\\
5 & 6 \\
\end{tabu}
\end{center}

\end{document}


Output:

• Your \hyphenation{...} is missing the letter g Aug 19 '12 at 4:59

1. As Lev Bishop already said, the words are different, g is missing in the alphabet word in \hyphenation.

2. If the text width is decreased, the first word is not hyphenated because TeX does not hyphenate the first word in a paragraph. Adding \hspace{0pt} helps here:

\documentclass{article}
\usepackage{tabu}

\setlength{\textwidth}{.5\textwidth}

\begin{document}

\begin{center}
\begin{tabu} {|X|X|}
\hspace{0pt}abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz &
abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz
\\
5 & 6 \\
\end{tabu}
\end{center}

\end{document}