My take on the answer to the OP question is "maybe" its main purpose has to do with the difference in dia of the barrel to the receiver at the rear sight location, and the raising of the sight base so the rear sight leaf - leaves are visible and the leaves themselves don't have to be as high and consequently are less prone to damage. The same reason the fron sight sits on a ramp.
Weight issues and fitting easement has led some to the island base instead of the full length rib or the quarter rib.

Von Gruff.