Probably it's because in Gen 5 we have 2 trio? The Kami Trio and the Muskedeer Trio (yeah, I know that one is actually a quartet, but only if Keldeo is included)
Anyway, another thing I want to know, what's with that old man near the Gallete stand that mentions about "shower"? What does he mean by that? Is there a meteor shower at certain times similar to diamond dust in Gen 4 and 5?