It is an old issue that was never corrected it seems. On Windows XP at
least, the unicode field is never filled correctly for the numpad number
keys. It makes thoses keys impossible to use for number input.
Here is an old e-mail on that topic :
http://www.libsdl.org/pipermail/sdl/2000-October/031169.html
I tested today with pygame and the latest SDL and in fact, the bug was still
here.