I got to agree with Sarge. Bad shot is the problem, not a super tough skull.
I've seen the same thing with 22LR Hyper-velocity bullets on woodchuck skulls. 99 times out of 100, brain splatter, that other 1 time, ricochet. It's just the coming together of the variables.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.