IIRC, even trained folks shoot low. The reasons being that:
1. They start to fire too early under stress.
2. They apply extreme pressure to the trigger under stress, yanking the gun down.
Because of this, some trainers say to take advantage of this in what's called the zipper technique.
In a training session, we had a drill where you stood in the center of 4 opponents (airsoft guns) and one would be the shooter at you. You had to respond. Many of the shots from trained folks where at the navel level or below. Ouch, they stung close up.