A lot of things in the anime don't match up with what's going on in the games. For example, Oshawott should've evolved by now, and Ash's Pignite shouldn't be able to learn Fire Pledge on its own. But if I were to make a guess, I'm assuming Piplup absorbed all of its evolution energy, then let it out of its body so it wouldn't have to evolve. That'd be my guess, anyway.
