Good question!
When we reload, we try to make the most accurate ammo we can. When we go to the range, we have a few bullets with different amounts of powder inside them. These bullets will be flying at different speeds. We are trying to balance power and accuracy.
So when we shoot at a target, we are aiming at a specific spot... usually the bulls-eye. We fire a few rounds, each shot aiming at the exact same spot on the target. It doesnt matter if the shot does not hit the bulls-eye, what matters is how close to each other the shots are. What we all strive to do is put 3-5 rounds on top of each other, or as close to each other as we can.
If I load a few rounds that have different speeds, and I find that the bullet going a little faster, or maybe the one going a little slower, or maybe the bullet I made a little longer or shorter is the one that makes the closest group on the paper; then I record my results and make more ammo to that specification. I now know that this is the most accurate ammo in my rifle. I may have shot 20 rounds and not hit the bulls-eye once, but you would see that I have groupings on my paper.
Once I know what ammo is the most accurate, I then calibrate my scope, and set my zero. I can then hit the bulls-eye when I want to.