I have a bunch of Glocks, 17's, 19's, and 26's, and they all "shoot left" for me, with the rear sight centered. For me to get POA/POI, I have the right edge of the rear sight on every one of them, on the right edge of the dovetail.
I put night sights on every one I buy, and have the tool, and I know exactly where to put the rear sight that I normally dont have to move them when I first shoot them.
Ive tried all the things the experts say Im supposed to do, change my grip, change my finger positions, etc, and they all still shoot left, if I dont move the rear sight to the right.
Odd thing is, Glocks are the only guns I have to do this with. All my other handguns, auto or revolver, all have the rear sights pretty much centered in the dovetail, if there is one. The Glocks are the only ones I have to change.
Personally, I wouldnt get to crazy worrying about all this. Just drift the sight to the right where you need it and move on. As long as you hit what you were aiming at, who cares?