That's a bit of a surprise, considering the oft-cited links between mathematics and music and the well-publicized math prodigies out there. One possible explanation might be that an apparent prodigy in math is something different from a prodigy in music; that fluid intelligence lets you pick up the technical skills to be writing symphonies as a teenager, but that writing engaging and expressive music requires an additional, different skillset that's more reliant on crystallized intelligence.

I'm not a classical music buff, but I've heard that Mozart's childhood work, for example, wasn't very good. Certainly not in comparison with his later stuff.