As for hits in the head, depending on area and force which is being used to strike, can cause major damage, ranging from getting simply knocked out cold for a few hours to permanent loss of functionality over body as result, for example, strike a foe in the jaw and ear area at the right spot and the it sends a shockwave through the bone to the brain and our ear (where our 'auto balance' is located) disrupting it briefly, which can put the person into dizzy state or knocks them out cold instantly, that's the main reason why boxers and mma fighters can just drop down after being hit once in the right spot, even though they are tough sons of bitches that can take a beating, but all it takes is one precise hit and these fellas drops down for count, and that effect can't be trained against, call it weakness of our meatsuits if you will, thats the reason why all martial artists will train to protect their head at all cost, there are not so many concussions our brains can take before those break for good, and we with it.
So, in a gaming sense, it makes sense to give 'head shots' higher damage rating than the rest of body.