[tex-hyphen] hyphenation different in xelatex?

[Karl Berry]

Here is a tiny latex document reporting hyphenation points in a given word:

\documentclass{article}
\usepackage[german]{babel} % same behavior with ngerman
\begin{document}
\showhyphens{Quellcode}
\end{document}

When run with latex or pdflatex, the (terminal) output is Quell-co-de.

When run with xelatex or lualatex, the output is Quell-code.
(The US English hyphenation is also Quell-code.)

Why?  A difference in allowed hyphenations between engines seems
absolutely wrong.

I could make guesses and/or spend (a lot) more time investigating, but
hoping someone else knows the answer already.  And, more importantly,
how to fix it.

thanks,
karl

P.S. Ironically enough, this came up in the first place because Herbert
Voss told me that Quell-code is preferable in German, too!  Oh well.
Quelle horreur de la code ...