The triggers could have been switched around at some point for some reason. Maybe the shooter wanted the left barrel to be the primary shot. I would wonder what choke is on that left barrel. Is it tighter than the right?
I have heard of some guns where the chokes have been reversed from what is customary (Right barrel or front trigger is tighter than the left). I have been told this is not very uncommon in europe where hunting driven game (coming at you) is more of a common practice. In that case your first shot would be farther away than the second. Hence the want for the tighter choke to be on your front trigger.
Can't answer for the case of your actual gun. But my guess is either specal order or a switch by a previous owner.
