well bryce probably leaves a bit of headroom for doing other things like word processing and internet browsing, because otherwise the computer would be completely unusable while rendering, because it would be 100% maxed out. When you play games you don't need that headroom, because the game is all-encompassing and you can't browse the web while in the middle of doom or halo. So IMO it's actually a nice little benefit that it leaves a bit of the CPU free for other minor tasks, if it didn't, you'd practically be without a usable computer for hours or days while your scened rendered.