Howler

What he said along with the fact that bullets leaving the barrel at different speeds are leaving the barrel at a different harmonic point. In other words, your barrel vibrates every time you shoot it, so depending on what point of vibration the bullet leaves the barrel, the groups will go accordingly. That's why when a person handloads, one load/speed may spray bullets all over the place, but change the speed/load with the same bullet which will leave the barrel at a slightly different harmonic point, and whaa laa, same bullet different speed prints a tight little group. And thats' the reason that bull barrels typically are more accurate(less harmonic vibration), all other things being equal.