Ok, I ran this by my good friend who is a forensic anthropologist. (No, he does not watch the show, but I'm working on him!) He said that during the first few months of the outbreak, yes, expecting skulls to implode like paper mache is unrealistic, but that after a year of essentially decomposing out in the elements, the bones throughout the body will become more porous as the skin rots away. Add to that an unknown pathogen that could speed the wasting process if the corpse does not feed adaquately, and easily perforated and/or shattered skulls are not only possible, but likely.
Not an expert myself, just relaying what the only expert I know had to say.
I bow to your guy's experience. The few bodies I had to deal with in that condition, I was too busy holding my breath, shoving them into the bag, and trying not to dribble the gooey stuff on myself to have much curiosity about the consistency of the bones. I know! I know! As a true zombie fan, I should have shown more dedication.
Can you all forgive my negligence? I was young and foolish at the time.