This helps most people:
Just kidding.
At one time, I owned 12 Glocks. I'm down to eight now. With all of my Gen 1 and Gen 2 Glocks, the sights are all centered and they shoot point of aim for me. I have large hands, and for some reason, 3rd Gen Glocks all shoot slightly left for me. The sights on my Gen 3's are all drifted slightly to the right.
I know it's me since other people shoot them and they don't have the problem I do. I have set a Gen 1 G17, Gen 2 G17 and a Gen 3 G17 on the bench in front of me and shot 5 shots out of them all. I ended up with 10 shots dead center and 5 shots slightly left. It's just the way the 3rd Gens fit me, or don't fit me for that matter. I don't have a problem with it, since after the sights are drifted, they hit where they should.
That's why I prefer Gen 1's. Glock had it right the first time.