Should I replace the rear sight on my revolver?

I just sighted in my Ruger GP-100 at an indoor range, and it kept shooting to the left so after turning the adjustment screw and checking it several times, the rear sight blade is way to the right and you can see the adjustment screw backing out. Could the rear sight be that out of specs. from the factory, or could there be some other problem? It's sighted in now but I don't like how far the rear sight blade is off to the right. Have any of you ever seen a rear sight so out of whack on a handgun before? Should I replace the rear sight? I have used the stock factory sight on the front, and also a Hi-Viz sight on the front. With either of those two front sights, I still have to adjust the rear blade to the extreme right to pull the point of impact to the right. I was considering getting a Bowen Rough Country sight for the rear sight on my GP-100, but would like to know why it's so far off in the first place.