This is because few manufacturers can afford making a true .410 action. As it is, most use the same action as they do for 12 or 12 bore guns. The key factor here is the distance between the firing pins. It is, logically, one time the bore dia. at breech add twice the wall thickness at breech. Baikal "big gauge" action, for example, has 27 mm between firing pins. If you try to fit a set of .410 barrels on this action, to have firing pins hit the middle of the primers, the barrels have to be spaced urther apart than what would be in a true .410 action. Then, to have the barrels hit the same POI, they have to be spaced apart at the muzzle as well. And this is what you see. It happens with O/Us too, only it isn't as conspicious.
