Right, we ran some tests between my 18" 870 and a buddy's short barreled shotgun (10") barrel and the shot pattern was also very similar in the amount of spread, though his patterned less uniformly with the same ammo (which may or may not be an issue with length). The shorter gun seemed more prone to throw fliers, but once again, that might have been due to ammo choice.
His gun, with a pistol grip, was a whole lot harder to shoot well, though I am sure it could be done. I had trouble hitting center mass on the target at 15 yards to get good pattern size measurements.
"If you look through your scope and see your shoe, aim higher."
-- said to me by my 11 year old daughter before going out for hogs 8/13/2011