Cool -- I didn't even consider the shape of the waveforms (sine-like for flute, saw/square-like for anything distorted) having an effect on the data, but now that you tell me... duh!
These lossless codecs work by attempting to predict the next sample from all the previous ones, and then encoding the difference between the prediction and reality. So a more regular waveform will compress better than a crazy-looking one. It's somewhat counterintuitive since rock music tends to be less complicated from a high-level human perspective.