"CMYK has a smaller gamut that should fit right into the RGB work space."
That's your problem. It fits fine into the RGB workspace. The problem is the translation back to cmyk. Since the RGB gamut is a better definition, the cmyk translation has to be edited back down, and it doesn't actually "remember" the original definition. Or in other words, a lot of cmyk colors can be defined by the same precise rgb definition. Photoshop doesn't remember the original, so it picks just one of many possibilities.
Think of it as a dictionary definition. If you take the definition of a word from the pocket Websters and put it into the full Oxford it fits fine, but the Oxford editors are going to add a lot of additional info to make the definition more precise. But if you try to take that Oxford entry and fit it back into your Pocket Websters you'll need to cut out a lot of verbage. You'd need to take some sort of manual steps to preserve the original definition so the translation back can get back to the original.
Moral of the story is that if you must convert color modes back and forth, you need to be careful to preserve your original color definitions.