Does the unit mount "plumb" to the barrel centerline, or does tightening the knurled mounting knobs skew it to one side? If the latter, some very thin shims on the opposite side might help, if you absolutely MUST have it shoot to mechanical center (why, I dunno). Does the front sight appear to also be "centered and vertically plumb" (I assume it's a fixed post)? Have you tried different types/brands of ammo? Just too many variables to worry about the rear sight alone being the problem at this point, IMHO.
The actual grouping (once adjusted) looks pretty decent, and if you're happy with that and STILL have adequate windage correction available in both directions for any reasonable conditions/distances (and it appears you do…), I'd not worry too much about it. That's what windage correction is for…to adjust for myriad differences in the weapon/ammo/ambient conditions possibilities. Take it outside to the likely ranges you're going to use it for and see what increased range does to the polynomial equation. If you're able to hit where you aim at 1-200 yards, be happy.