Not exactly a correction - more of an observation, but....

It is true that the path of the bullet from barrel to target is an arc due to gravity pulling the bullet down towards the ground. However, at 15 yards the amount of drop due to gravity is going to be very tiny. At that short of range the bullet's trajectory is going to be almost perfectly flat.

There is also some effect from gyroscopic oscillation. The bullet is spinning at high RPMs and minute imperfections create both a slightly out of balance wobble and aerodynamic forces that disturb the bullet's flight path. Think of how a spinning top does not stay in one place but moves around in little circles (oscillates). The bullet does much the same thing - only in 3 axis of motion (left/right-up/down-forward) vs the top's 2 axis of motion (left/right-forward/backward). Kind of like how a spiraling football doesn't fly in a perfectly smooth arc - but instead sort of corkscrews a little bit. But that is also only a minor variation.

The big variable is the movement of the gun under recoil. Barrel rise, and "torquing" sideways due to grip, etc. The fact that the shots are still grouping despite the difference in POI it the key proof of it. It is totally repeatable.