Answer:
The answer is supposed to be simple, but all of your ideas are valid.
When soldiers wore cloth helmets any round fired at their head had a very good chance of penetrating the cloth helmet with enough force to kill said unlucky soldier. When they switched to armored helmets, said rounds were slowed or deflected resulting in injuries, but not deaths.
In other words, the same amount of people were being shot in the head, but more of them were only wounded, thus increasing the number of head injuries.
No idea if its true, but that’s the answer you’re supposed to come up with.