It makes sense to package the two bundled tapes together. If you only own MP-13, you get all the tapes plus a spare Laserbeak. If you own MP-13B you get all tapes plus a spare Ratbat. Plus, as I said, that way they can get more use out of the (unique?) Rat Bat mould. G1 Rat Bat was never re-coloured as far as I'm aware, and Overkill, etc. would require new designs and toolings. It's an easy way to make money at minimal cost.