[a1k0n]

Not at all... "documents" were the user's play history and "words" were artists. So if you played one artist twice and another artist once it'd be like a document that says "artist1 artist1 artist2". The assumption is that document topics are analogous to music genres, and each artist creates music within a small set of genres each user prefers music within a small set of genres.

[dhammack]

Ah, that explains why the groupings are so good. That's cool, I hadn't thought about topic models outside of NLP before.