Some pretty expensive single-barrels don't always shoot to POA (or "point-of-point") either, especially with a wide variety of ammo in a relatively lightweight gun.
In my Benelli Ethos 12-gauge, Federal's Prairie Storm 3" loads shoot noticeably higher than typical 2-3/4" loads. Once again, you never know until you pattern them....
In all likelihood due to the increased recoil and therefore muzzle rise while the shot charge is travelling down the bore. I mentioned this a few days ago, as well as suggesting to the OP that he try a few different loads. .
