Smogon does that to every mon that have different forms like Kyurem and Keldeo. If a mon has more than one form, megas included, smogon counts it as a different Pokemon, mostly because they have something to differentiate themselves from the base form like how White Kyurem plays differently from Black or regular Kyurem
Wormadam is no exception. If you take a close look, all three Wormadam forms have different types. That alone is reason enough to count them as different mons as they don't play the same way
