I believe the "Trigger Arrangments" were actually set when both bbls were cylinder bore, so the shooter would have determined which one to fire first. Quite obviously the right trigger fired the right bbl & left , left. One would really have to guess as to why the right one was chosen to be the forward one, but traditionally over the years it has been the one assumed to be fired first. It is also noted that most non-selective single triggers are made to fire the right bbl first. I don't think the driven game thing holds much water.
