Originally Posted by WildRose
The most likely reason is that it's due to a change in your cheek weld and eye,sight alignment.
I agree, changing your cheek weld will cause vertical changes in impact. So will putting the butt plate higher or lower in your shoulder. So does the amount of vertical pressure under the stock's forend on its rest.
But I disagree with a change in eye, sight alignment. With a scope focused at target range, you eye can be anywhere in the exit pupil's cone of vision as long as the reticule's on target. People shooting benchrest rifles in free recoil don't touch the stock at all with their head; their eye moves around a bit relative to the scope's optical axis for each shot.