@quintus : It's strange that you managed to get 31% if it was with the first colony level. Did you cheat to get a colony ? Which level ? Or did you get conversion without hellenic colony ? Poleis don't convert culture, at least in theory and EDB.
@Thoras : Did you upgrade it to a polis or to a metropolis ? Sinope starts with 60%, so you have the 49% for polis, but not metropolis. If you got a metropolis it's strange.

The EBD line you quoted is indeed what i'm talking about, and in EDB the only buildings spreading hellenic culture are colonies and bosporan governments. I don't understand how it still increases...

I read that in Britannia, spies and geenral's management spread culture. Could you test and check if a province without governor and no spies still spreads ?