When I ran CCleaner (1.00.077 Beta) on my Mac today, I became curious about the following "Caches" under the Miscellaneous category, which found about 3.5GB of data:
Presumably various system caches or other program caches not covered by System categories, perhaps one of the developers can chime in (I don't have a mac, and I don't know if it operates like the windows version where you can see within the program given the right command.