POI changes due to magnification adjustment is not unusual. The amount you experienced is on the larger side but still possible depending on the scope. About three weeks ago I went through this same scenario with a gentleman at the range. Every shot on 9x was exactly where it needed to be, drop it to 3x and it would shoot high about 3". It is most likely the scope moving the reticle that much when you change the magnification, some may be the fact that on 2x you may have a harder time holding true center.