Lastly, I hate to say this, it could be completely shooter-induced. First shot occurs right after the stabilizing thumb has wiggled around, and the first shot "seats" the gun completely in the palm, subtly changing the impact of even the most consistent shooter. Might even be a touch of anticipation/flinch that is ordinarily compensated for consistently, but that is nonetheless different on that first shot of a set, again, even for the most practiced shooter.