Gus Fisher has commented that he believes it's the barrel whip........harmonics placing the barrel in a whipping motion such that different weight bullets leave the muzzle at different points in the cycle. My NM has a 3" different POI between 168s @ 2600 fps and 147s at 2750 fps. The difference is mostly vertical but also slightly to the right. Bedding the receiver in a very stiff stock and using a unitized GC will diminish this somewhat. A heavier barrel will also. The heavier bullet has a higher POI.