Ok I understand that no shotgun throws the same pattern twice, nor is the perfect pattern ever repeatable. What I'm saying is that there are no major holes in the pattern, nor any major clumping of pellets. AKA even pattern. For the sake of the question, let's assume that all 350 pellets in both the 12 and 20 gauge loads are inside the 30" circle.
Now, because a 1 oz payload in a 20 bore is in a longer shot column as it sits in the shell and in the chamber and as it goes down the barrel, how is the pellet swarm different as opposed to that same 1 oz payload in a 12 bore which is not as tall in the shot column while in the shell and the chamber and the bore? Because of the larger bore size, does a 12 gauge have a shorter (from first pellet to hit the patterning board to last) shot string?
