Yes, but the "back of the head" proponents also cite other witnesses such as those at the autopsy, the nurses who "cleaned up" JFK for the casket, et cetera. If we didn't have the physical evidence - the x-rays and photos, the films (plural: notice the Nix film?), the blood/brain matter landing in front of where JFK was - the "back of the head" side would have a very strong case. But that requires ignoring all of this other evidence. Or making the usual conspiracy response: it's all fake. Which, as we know, they do.
It is puzzling. This has probably been theorized before but the following could be an explanation for the differences in observation.
In the Zapruder film, we can see that the top of his head just opened up. The autopsy top of head photos show that damage. The rear-head photos show that triangular flap of cranium hanging off at the side. In the lateral x-ray photo, we can see that the rear-top of the skull is detached (put into place for the x-ray).
We can also see severe lines of fracture extending down towards the occipital region. It could be that that rear portion was also loose and it flapped open as Kennedy sank down in the limo and also on arrival at Parkland. Thus explaining what Clint Hill and the nurses saw.
In the trauma room, with Kennedy on his back, the rear flap wasn't as easy to see. It had perhaps been pushed closed (by chance or on purpose). However, the damage to the rear was observed in that room.
For the lateral X-ray, and rear-head photos, the flap was pushed back into place (to dispel any suspicions that it might be an exit hole of a shot from the front).
