Same here, one reason I leave my scopes on one magnification (usually the highest for me. just because I'm used to it). The difference can be minor (less than an inch) and not anything like 12 inches at 75 yards.

Most standard eye relief scopes are set to have the least parallax at a hundred yards. Long eye relief scopes at 50 yards. Some are adjustable.

Parallax has something to do with how the target, front lens, rear lens, intermediate or reversal lens/reticule and your eye align. The light in your scope doesn't travel in a straight line anyway, seems counter intuitive but fact. The four (most often) lenses have to align, when you increase magnification some lenses move. The likelihood of the bent light not aligning in exactly the same plane increases. Better people than I have tried to explain this and usually needed a book to do it.

One major variable can be if your are crowding the scope. Most standard rifle scopes are designed for 3 3/4"-4 inch eye relief. Crowding the scope can really magnify the variables. If you see a black ring around your image in the scope you are to close.

Make sure nothing is touching your scope but the rings.

Make sure all your mounting hardware is tight. I've had ring screws come loose before.

The reticle itself is on a separate lens (prism) located where your windage and elevation knobs are located. Not beyond the realm of possibility the reversal lens (turns the image right side up and has the reticle on it) came loose or your elevation and windage hardware came loose. Not uncommon for a dropped or damaged scope. I've seen people do a poor job of mounting a scope and running out of windage or elevation and stripped the threads trying to force the elevation or windage knobs, beyond the end stops.