Picher, there's several things that'll effect the vertical shot placement between different bullets leaving at different speeds as well as a given bullet leaving at a given speed.
One is the amount of muzzle axis change in the vertical caused by the rifle's recoil while the bullet's going down the barrel. That'll vary with how hard the rifle's held into ones shoulder and the resistance ones body has to that recoil. This is the reason why several people will have different sight settings for a given load's zero for them; we all don't hold rifles the exact same way.
Another's where the bullet leaves in the muzzle axis angle while the barrel's whipping like a wiggled fishing pole; slower bullets leave at a different angle than faster ones. Many times, the bullet's exit point in the muzzle axis angle help accuracy; slower bullets leaving when it's higher on an up swing compensates with bullets leaving faster and lower on that upswing.
Adding another variable to the barrel's wiggling such as different amounts of external pressure on it at different points such as those applied to the stock that's touching the forend compounds these issues. If the fore end's not able to touch the barrel, then the barrel wiggles more consistantly from shot to shot.
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master