Might be an addressing limitation, though I'm still inclined to think it's just ASUS doing wierd stuff with their bios or perhaps trying to push their P106 cards (there aren't a lot of folks that MAKE the "mining specific" cards).
12-13 is an oddly specific number - ASRock and ASUS have 13; Biostar and AMD have 12; users with PCI-E multipliers get 12 as well, so I believe that there is an address limitation indeed. Mining cards count as a compute accelerator so they don't take up VGA address space, this could be the reason we can mix mining and regular cards on this motherboard.