Lots of explanations that are saying essentially the same thing.
However, if the proper sight picture is on the target when the gun goes bang, you hit. Or if the shot goes somewhere else, it's a sight problem.
A friend was shooting low, as usual. He said, "jeez, the sight was right there when I shot." I explained that it was not. "Your sight was right there when your brain said shoot. By the time you shot, it was not because you gave up and quit. Keep the sight on target until the shot breaks. Do that by CONCENTRATING on it. Like I've told you a hundred times (that kind of joking banter that goes on with friends)."
He'd shoot well for a while, especially quick multiple shots. Then he'd start going low again, especially when he took more time.
Then, when I'd shoot low, he'd tell me the same thing
. And he was right, though I knew what happened. We all do it. Knowing what we did is what's important. Not easy to always concentrate on the front sight.