Confusing high temps..

So i ran a stress test for over 3 hours on my comp and it never went over 50c.Why is it that when i play BFBC2 it's maxing near 55c.I have no idea, can anoye shed some light onto this plz?

Phenom ii x4 965BE
XFX 6870
 
Solution
When you play BFBC2 it also works your GPU in addition to your CPU, so the ambient temp in the case also goes up, causing everything else to heat up, Prime95 only stresses the CPU.