It realy isnt. Each game uses teh GFX card in a different way. While Crysis didnt artifact it could be because it plaecd more stress on the CPU than the GFX card. Or vice versa. while FEAR being a older game probably taxed the CPU more and caused the OC to affect the way the GFX were showig and caused the artifacting. Hence why just taking it down 10 stopped it. Gave the CPU/GPU more time to work together instead of against each other.
Of course i could be completely wrong with my guess and it is just a fluke that FEAR didnt like it cause it is a older game and cant handle that kind of speed itself.