Why are break barrels hold-sensitive?

The pellet is still inside the barrel while the piston slams forward, so the gun moves a measurable amount during the shot cycle (lock times of 5+ milliseconds for spring guns, vs about 2-3 ms for centerfire rifles).

Any inconsistency in how you hold or rest the gun changes where the muzzle is pointing when the pellet finally exits, which throws point of impact around. The fix is the artillery hold and consistent technique, not gripping the gun harder.