I will try to explain cant quickly. Using extremes.
The bullet leaves the barrel 1.5”-2.75” under your scope. Most rifles are set up between 1.75-2.25”. That is significant. We are not shooting laser beams. We are shooting a bullet that leaves the rifle and immediately pulled by gravity in an arch until it stops.
If you took your rifle and laid it on its side with a 90 degree cant and dialed 8.4mil on your scope for a 1000yd target. Would you expect to hit your target? Absolutely not. Even if you had perfect fundamentals and great npa you will miss your target by a lot.
Well now break the same shot with a 45deg cant…you will still miss by a ton. Now try 15deg cant. Still missing. As you get closer to 0deg can’t you will start seeing less drift on your POI.
This is just basic physics.
But let's distinguish here:
1. Scope
2. Barrel
Let's say you are a funny guy and mount your scope on the side of the rifle but perfectly level to the ground. You zero it in. Now your scope is 90deg canted to the barrel. I bet you'd still dial and shoot correctly if you could somehow get into a position behind the scope -- let's say on a shooting bench that completely locks in the rifle. Up is still up, left is still left.
Or different example. Let's say you love holding your rifle 45deg turned due to a weird injury. If you leveld scope to rifle, your windage adjustment would be 50% elevation adjustment and vice versa. You could still dial but it would be a pain -- two clicks up and one right to hit one click up, say.. -- but if you now made the reticle and scope level to the ground instead of the rifle, dialing would be normal again?