For some reason, it tends to bounce around the abdomen a bit, *nicking* internal organs.
Skull shots are always unpredictable. For some reason, some people have skulls as hard as river rocks. Others are quite brittle.
Attempted suicides are the most common, with some rounds of different (.38 and smaller) rounds bouncing off. One guy shot himself on the side of the head, the bullet penetrated the skin, but not the skull, then traveled under the skin over the top of the skull to exit through the skin on the other side. He shot himself 3-4 times this way, until he was in such pain that he had to stop.
You might imaging how pebble-on-a-cars-windshield fractures on the skull where there bullets originally hit, must have hurt like a son of a gun.