delete-char deletes marked text sometimes and sometimes not

[Mathias Dahl]

Try this in CVS Emacs:

 $ emacs-22-xyz -q

Type some text and mark it by temporarily activating
transient-mark-mode (C-SPC C-SPC). Press C-d (delete-char). This does
nothing to the text, which was expected (at least for a long-time
Emacs-user... :)

Now type some more text and this time mark the text using the
mouse. Now press C-d again. Voila! Text deleted!

What is this magic? Some "stuff" in the text properties, or what?

/Mathias