One word can account for part of it...TorqueDoesn't the laws of physics work equally in horizontal axes besides vertical ones?
Why else would a left handed shooter shooting the same rifle slung up in prone have his windage zero the opposite side of the sight's bore sight setting than a right handed ones zero?
Why else would someone shooting prone as above see horizontal shot stringing with his front elbow shifting left and right in different positions under the barrel?
Why is a zero obtained with a rifle shooting prone horizontally changed when the rifle's zeroed as it rests on bags atop a bench and its butt against the shooter's shoulder?
The horizontal position of the mass center holding the rifle causes different amounts of barrel whip in that axis while the bullet goes down the barrel.