Sounds like the scope and/or rings were settling. It should not be that far off, as you say. I would loosen the rings and roll the scope back and forth to see if the croshairs stay on target or not? Or better still, make a scope roller jig with wood and nylon round-head screws (image below). You adjust the settings until the crosshairs don't move when you roll the scope around on the support. That centers them. At that point if your rings are centered and parallel to the bore, when you mount the scope, the 100 yard target should be impacted low by the amount the scope axis is above the bore plus the bullet drop. Even with a 100 grain bullet at 2800 fps, that drop is only about 2.3 inches. If your scope axis is 1.5" above the bore axis, then this is a total of 3.8 inches low.
I would just try grouping it like that with no further adjustment. If the shots move around after that, then you have something loose. Unfortunately that something could be inside the scope, which would necessitate a warranty return. Pulling the scope back off and rotating it in the jig again should tell you if its adjustments have slipped? Alternately, if you have a laser bore sighter you should be able to see the laser spot shift relative to the crosshairs if the scope is shifting?
