Everyone shoulders and lines up behind a rifle differently. Where you place your eye in the eye box and vision all play a role in where a rifle impacts a target. Do you shoot both eyes open or one eye closed. It is all about consistency and form and how the rifle is set up to an individual. As long as you do something consistently you can cover up a lot of bad form.Same experience here. I'm drilling bullseyes with his rifle after mounting a scope for him. His group was darn near 3" right!
I shoot off hand with a slight cant to my rifle. I am anal about my set ups. Use levels, a tape measure and more to make sure all of my set ups are consistent from one rifle to the next. I take the time to adjust the diopter to my eye. I shoot both eyes open. My dominant eye is 20/10, my bad eye is 20/35 on a good day. I have a friend who shots both eyes closed I think and sets his junk up with tinker toys. He kills everything he aims at. I can't shoot his rifles for beans. And he can't shot mine.
One day at the range he wanted me to shoot his gun at a grand. Mainly because he wanted to shoot mine. I set up behind the rifle and let it bark. Missed down the right side of a silhouette. He laughed! You missed! I missed two more times. I check my wind and settled in, let the bubble on the scope tap the right side of the level and squeezed one off. Center punched it! I put three in about a 3 and a half inch group.
Everybody sets their rifles to them. You really see the difference when using a bow. Some people put the pin on the target and cover it up. Some like the pin to point to the target. Others let the pin rest just under the target. Add in different anchor points and you can have two identical shooters with perfect form and they won't be able to shot each others set up.
There is a proper way to do things and proper form but this ain't rocket science and just the differences in body type, alignment and set up can make a huge difference! Three inches right and high at a hundred is a miss at a grand. It is important to make sure the rifle is set up to the hunter hunting with it and not someone else.