The frame of the rifle is large enough for 12 gauge shotgun barrels. The .458 barrels are close enough to "fit" together nicely but for the .375 barrels they have chosen to keep the double rifles weight down by separating them. This also allows the barrel set to be able to be self-regulated and makes the rifle very versatile. With the scope on the .375 it would make a dandy plains rifle and a big bore barrel set in .458 a DGR as well. 12g barrels instead of the usual 20g set makes the shotgun barrels more versatile too.

Usually most doubles like this are made on a 20g frame.

Gryphon is very right. This set makes a very nice one gun "world" hunting rifle. Even though it is in under and over configuration which many here do not like.