The principle is best summarised by one word - consistency.
If firer, aim, rifle, ammo, weather conditions etc are all precisely consistent from shot to shot to shot, the bullets will almost cut a single little hole.
Rifles, sights and ammo are now amazingly consistent, and of high quality. It's the shifting vagaries of the firer and the weather that cause the problems. And just as well, too. Who wants to score a possible every time? Boredom . . . . .