How the respective grips fit your hand, the mechanics of the trigger squeeze per the grip and your anatomy, and the anticipation of the magnum recoil can all affect the results.

I found in my brief and frustrating foray into Bullseye competition that the fit of the grip to my hand and the positioning of my finger in relation to the trigger pull mechanics on certain guns caused grouping in specific places. "Heeling" the gun in anticipation of recoil produces a like result.

Or, as I was told, "it's probably the loose nut behind the grips".