If you have an accurate scale, simply weight a few rounds of factory ammo (not match ammo) and you will see an amazing inconsistency in weight.
More important than that...a re-used case has expanded to the bore in which it is fired. After that expansion has taken place, no energy is lost when the shell ignites during future usage. It is only with these casings that sighting-in should begin.
Of course, only bolt action rifles are able to advantage of this property as magazine fed shell casings need to be re-sized prior to reloading, eliminating the custom fit.