As Saul said in Chapter 1, "Who said they were dead?"

I think they are like 28 Days Later infected, in that they are still alive but have a disease. Also in Chapter 3, Saul points out that some people are killed (and eaten) while others are turned and wonders how the Zed's choose.
While head-shots are most effective, there is evidence that at least certain types of zombies will die from damage to the center mass. In 12-4, Burt kills the Jumper with a shotgun blast to its chest and in 13-2, -3 he uses a SAW on the mass horde surrounding the firetruck. No way he was getting all head shots with that. The last one I can think of is in 7-3, where Lizzie describes Michael and Saul as blowing holes in the two zeds that had come onto the roof. I think one bullet to the head will drop them, but cause enough damage to the center mass and they will go down. This I think points to some kind of parasite or microbe or something controlling the body, in that it can make them tougher but enough damage will take them down.

Also, the Behemoths have something special that no other zombie has, with their thick hides and they don't need their frontal lobes (how the one in 10-3 survived Riley's arrow). However, Kalani was able to slay two by shooting them in the mouth. They still must have a soft pallette and the roof of the mouth would be much closer to the center cortex of the brain than going through the eye. The brain-stem part (I'm currently blanking on a lot of the scientific terms, forgive me) of the brain is the area most zombie experts agree needs to be destroyed, since it runs the body functions. That is my explanation for why Kalani could shoot Behemoth's dead through the mouth when Riley's arrow failed