I believe Toby has looked at a wide range of music, and considered many aspects - such as middle passages, slow starts etc. Although the algorithm displays a real time approximation, it must "see" the whole track before offering a definitive BPM.

I'd try it with some Jazz, but Toby is on holiday and we compiled out his BPM stuff in his absense, because it scares us

I'm sure you'll see lots more of this feature soon.

Rob