Patterning on paper gives you a 2D view of where your shot pattern is. The problem is that you are shooting an object in 3D.
Perhaps I should have said 'effective pattern', the size of pattern at which a bird will be solidly broken (that is not a cheap chip caused by one of the extraneous flier pellets).
Through extensive testing (shooting skeet targets) with 12/20/28/.410 at targets from 5 yards to about 25 yards, I have determined that each gauge has a 'sweet spot'. If you put this sweet spot through a target, you will obliterate it. The size of the sweet spot gets progressively smaller as the bore size and payload are reduced. Of course it is also extremely tiny between the muzzle and about 5-7 yards, and it will be lost to the natural spread somewhere after 20 yards.
For anyone who thinks a 12ga effectively patterns the same as a .410 (let's go with the extreme example), sit on station 7 of a skeet field, and shoot at only low birds. You will see that the effective kill zone on a 12ga is significantly larger than on a .410.
If I was to restate, I'd say the sweet spot on the steel 12ga ammo fell somewhere between 20ga and 28ga. Some of the chips that you'd normally get with the 12ga just weren't there. The hits in the sweet spot were very hard.
Brad.