Somewhere along the line, the rear signals were changed out for a dual element lights. This would allow the bike to have running lights in the rear. They probably just put a set of front signals on (which are dual elements). Since you have one of each, I would say that after doing this modification, the bike was dropped and busted the rear signal. Since the previous owner didn't have another front signal to put in its place, he put the stock signal back on.
I honestly don't see any problems with this...other than the lights/bulbs are not the same. One side will be on while running and the other will not. As far as I know, there are no laws saying that if you have running lights in the rear, you have to have both on.
Personally, I would replace the single element with a dual element fixture to have the added visibility at the rear of the bike.