I am a native speaker so I don't really care about the number of tones, I was just citing the link. You can speak to me in monotone and I will figure it out from context just like how you can learn to understand someone with a strong accent.
My theory above is that a fixed number of tones is a linguist madeup concept from data that happens to fit the clustering of the tone graphs. The real tone is a distribution on a per word basis that you learn from natural variations from the other speakers around you. Do the 6 or 9 or 4 tones sometimes fit these distributions? Yes, but it's still just an approximation of the real variations that natives use and shouldn't be treated as an authoritative pronunciation.
My theory above is that a fixed number of tones is a linguist madeup concept from data that happens to fit the clustering of the tone graphs. The real tone is a distribution on a per word basis that you learn from natural variations from the other speakers around you. Do the 6 or 9 or 4 tones sometimes fit these distributions? Yes, but it's still just an approximation of the real variations that natives use and shouldn't be treated as an authoritative pronunciation.