They clearly understand that, it's just the 2.1 is labeled O3DS (it's not wrong) when it's the only file there is.Don't get used to this though.
I would recommend you to use the n3ds files with new 3DS systems and o3ds files with the original 3DS if you don't want any surprise later
That can get confusing for people that don't think offhand "2.1 is O3DS only".