For the record, UFC doesn't make the rules. It's the athletic comission of whatever state the event is being held in. I agree though, that it would be nice if the refs were more consistent but I'm not big on the "back of the head" rule when the "victim" is constantly moving into those shots.
The back of the head rule should be something like not punching the back of a head while a guy is trying to take you down. When you have the guy in mount and are trying to punch his face, but he's moving back and forth - should not apply.