All gun barrels undergo a resonant vibration when shooting. This means the barrel is whipping - sometimes in a rotary motion - on each shot. If the bullet exits the barrel closer to the Peak (high or low and right or left) then small variations don't change your point of impact. If they exit in the middle of the whip - when the barrel is moving faster then even small variations can change the point of impact a lot.
When I work up a load I find the most consistent round over a few months of testing and tuning the loads. Then I get the chronograph out and check the velocity so I can work up ballistics tables. That is how I get sub MOA groups (sometimes sub-caliber) consistently. |