For want of a weapon, I have been roaming around the island beating zombies to death (well, more death) with my bare fists. I've noticed while doing this that sometimes when I attack a certain zombie, I will gain some experience (usually 7 XP) with each attack I land on that same zombie, along with the bonus I get for breaking it. This only seems to happen with certain zombies, and it doesn't seem to be dependent on whether or not I'm attacking with an actual weapon or going melee. What determines whether or not a zombie will give me experience each time I hit it?
If it makes a difference, I'm playing Sam B., the rapper. He is a true bro.