I think that what some people here seem to forget is that neither an RGB-value nor a CMYK-value really represents a physical color. They are not device independant.
To get a physical colour from an RGB or CMYK value you need a colorspace. The colorspace is the cross reference between numerical values (RGB or CMYK) and physical colors. I think that is part of the reason that the values change. Also CMYK colors can be adjusted by for example under color removal without losing their hue. /Peter
