Funny, I was thinking of coming to this forum with a similar question after what happened to me the other day:
I don't have great eyes and I find visibility in this game to be terrible, so I usually go for a x2.5. But then yesterday I picked up someone else's weapon with a x1 by mistake, and went on to make two difficult shots, maybe 30-40m enemies running perpendicular to my aim. And the thing is, I really didn't feel I was aiming particularly well, it felt like I was being helped by the game quite a bit.
I wanted to test more but as I said, I can see much in this game, so I quickly dropped plans to play more with a x1. But I do have a theory:
There are only so many pixels on your screen, and each is a place where you can put your aim. But where the perfect aim would be could very well be in between two pixels: the game doesn't have much of a choice but to give you the hit on either one, or else it becomes unplayable. But when you zoom in, all of a sudden you have a lot more places to put your aim: you will be required to be more precise to get those hits.
Of course in reverse, it's much harder to see what's going on. I think x1 optics are particularly good with controllers, perhaps less so with m+kb.