My guess is that the conscious mind is overriding the trained habits. It's a control thing we humans grapple with: we want a certain outcome, and we know the individual factors needed to get that to happen, so we try to directly control all the elements. Instead of relying on training and muscle memory and instinct, we sometimes try too hard to be in control and mess it up.
I've noticed myself doing this in all sorts of activities, including but not limited to shooting. I think this is a major contributer to that accursed "beginner's luck" that we've all seen, where the new guy beats you in a game of clays or pool or whatever. He's not trying as hard and he does what comes naturally; it works out well, so he keeps doing it instead of worrying about his technique or whatnot. Then he learns the "right way" to do it, and suddenly he can't shoot straight anymore, till he's gotten to some particular level of proficiency. Or maybe that's just how it happens with me
I suspect that, after a person suffering from the OP's condition gets past the point of being a casual or amateur marksman and really starts mastering their weapon, the difference between their fast and slow fired shots will be far less noticeable-- at least I hope that's how it works, because I'm one of those suffering right now
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I'm not saying that anyone doesn't know how to handle their firearms or is a bad shooter, please don't misunderstand. Just that there's a reason my cop and marine buddies have much more consistent results than my regular-joe shooting friends. ....At least the former marine MPs who trained on the pistols. I don't know about the rest of them, really.