Couple of thoughts. Zero the rifle properly with the ring in a ring, and then the post at 6 0'clock, or, perhaps the BUIS you are examining wasn't made with the thought in mind of ring in ring sight alignment.
On the HK it works! Really.
On the actual hk sights the front sight is fixed with the post exactly in the middle. It works great this way, but you need a rear sight with windage and elevation adjustments. When the front sight is adjusted for elevation the post isn't in the center of the circle and therefore your aim is off. As shown in my picture when the 2 circles are centered but the post is low or high the point of aim tends to go out in a straight line almost paralel from the barrel.