Just a guess, incoming birds one shot then reload so you have both barrels loaded for the next pass top first would be an advantage on O/U guns. Only have one O/U & compared to my SXS guns the bottom barrel reload always seems akward. I read about recoil on 2nd shot bottom barrel advantage, I prefer to remount the 2nd shot, which makes it irrelevant.

Point is different shooters see different advantages & no doubt Boss built whatever the buyer wanted

Boats