2 3/4" shells in 1914 do not = 2 3/4" shells 20 years later. The Super-X resulted in an increase of service pressure. I would guess that Savage had determined their old 12's were good to go with the new, hotter shells, but did not want to risk additional pressure increase from a longer shell in a shorter chamber. They apparently felt that lengthening the chamber was a good way to tell Fox owners that their guns could handle the new shells.