This is actually a VERY common occurrence. We all like to believe that our scope mounts/bases, rings, scopes, erector tubes, barrels, and actions are all machined perfectly, but such is not the case. Lateral "drift" is very common. We sight in at 100yrds, essentially matching up an intersection between the straightline of the bore, and the straightline of the center of the scope. If these two lines were perfectly matched, the shots would track perfectly straight, showing no windage shift across different ranges.

However, we know real world weapons are NOT perfectly machined, and we know that our bore, action, mounts, and scope will not all perfectly line up on the same centerline.

So what we're dealing with is a long skinny "X" with the centerline of the bore/bullet's flight path tracing out one line, and the centerline of the scope tracing out the other. Granted, poorly stabilized bullets will actually CURVE during flight, but for the sake of simplicity, we can't always account for that mathematically.

Over short distances, typically this isn't too noticeable, but over LONG ranges, a load might be an inch or two RIGHT at a shorter range than zero, then an inch or two LEFT at a range LONGER than zero.

However, if you sight in at 25yrds, your "X" becomes shorter/fatter than it might be if you sighted in at 100yrds. So if you're on target at 25yrds, then 1" to the right at 100yrds, that's 4" to the right at 300yrds. However, if you were ON at 100yrds with the same deviation, you'd be off by .25" at 25yrds to the RIGHT, but then only 1" at 300yrds. (Granted, that's 1/3" deviation between the bore and scope centerline, which is exaggerated, but you get the picture.)