This is "not" uncommon. As was said, it's more than 99% about the difference in "people" and less than 1% in the gun/scope ect....

I can't count how many times i've seen this before, and it's why i would NEVER sight a rifle (or any thing else) in for another person. I always just get it close, and then "insist" that they shoot it while i make the final ajustments.

Drilling Man