As PM says, cant is an issue as it's also an angular measurement. If you have a 2" high scope on a rifle held on its side and zeroed at 15, (which of course you wouldn't but go with it) at 15 yards you'd be spot on. At 30 yards you'd be 2" off on the horizontal. You'd never have your gun on its side, but any angle you do have then this will tend towards this effect. Cant occurs in 2 ways, one is as PM says where you mismatch vertical x hair and ret, but shoot correctly, the other way is where you line ret and action up perfectly and lean the gun when you shoot, To give you an idea how