It bothers me too. The dead should have to receive significant head trauma to their brains in order to be "killed". I am thinking anything from a skull bashed in by blunt object, or penetrated from gun shot. These little random small punctures through the chin or even the side of the head just doesn't feel right. Also, the strength of the skulls seems to vary between butter and jello when they are doing stabbing them in the head. A teenage girl just took like 30 undead by herself, but yet a few episodes ago we saw Daniel and his men with the water track drive off in a hurry and fear because there were like four walkers half a mile away coming down the street.