It's been my experience that apparent POI shifts when drastically changing magnification can usually be traced back to incorrect parallax adjustment. The numbers on the AO end (yards, usually) do not often match real world parallax adjustment.
So, if your parallax adjustment is "off" (but not readily noticable) at one setting, changing the magnification also normally causes you to assume a different stock-weld, which compounds the parallax error.
I helped out a guy at the range a couple months ago who was having an apparent shift in POI of almost a full inch at 100 yards when changing from 4X to 12X.. He was ready to toss his hi-$$ varmint scope. I adjusted his parallax setting for the distance he was shooting, and he immediately decided his scope was a pretty good one after all.
Mike
