The raw images are exactly what your cameras sensor recorded, without all the post processing the camera does to turn it into a pretty snapshot. Instead, you get to do the post processing on your PC. How you do that, and what results you get is entirely up to you and the conversion software you use. This doesn't actually have anything to do with CHDK. If you can't get results equivalent to the the cameras jpegs, you are doing something wrong.
Canons firmware does a pretty good job most of the time, so the small bit of extra quality you can get doing the processing yourself is frequently does not justify the effort.
