So instead of having a neat more-or-less integer-based harmonic series like a 1D resonant object (i.e. a string) they have multiple complex resonant modes which are triggered simultaneously, and which decay at different rates.
So they're semi-pitched. There's usually a fundamental, but the other frequencies can be almost as loud. So if you tune the fundamental you can still get dissonance with the other modes.
All of this makes the idea of tuning a bit and miss.
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