Illustrator Colors

I've encountered this "situation" several times in different versions of Illustrator. When I'm working on a vector artwork, I assign CMYK colors b/c I would eventually be outputting it in CMYK often. So I work on the artwork, save then close the file. HOWEVER when I open it again, the color values change!. For example (for discussion purposes) I assign 100% Black, 0% Cyan, 0% Magenta and 0% yellow to a design element before I close it, when I re-open it at a later date, the values become, for example: 84.95% Black, 62.58% Cyan etc...etc...
Is this an Illustrator thing or a Mac thing? How can it be fixed? I've seen it happen on different Macs and different Illustrator versions since I'm on-call for different clients most of the time. They call me and I do the work in their offices. The color shifts are very annoying to say the least.
Many Thanks in advance!

When you create the document are you selecting CMYK? I opened a new document and when I used RGB and filled a box with a cmyk build it converted to rgb when I reopened it. When I started with a CMYK document it was fine.

DUH!!!!!! where did my brain go???? hahahaha of course! Thanks Cris!!! for the longest time it never occurred to me!

Thanks again and more power to you!