[Col] Can the native settlements upgrade?

[Endim_Analys]

Hi, I love the game and comes back to it regularly every few years. During my most recent playthrough I was surprised to notice that the Arawak villages suddenly became cities of the same kind as the Incas some time around 1600. I didn't not the exact year and didn't have saves to go back to to check, so I couldn't see if it would happen again, but the status remained on reload at least. They still used the same face in negotiation but when I destroyed their cities they were identified as such and yielded thousands of gold each. Is this a bug or can the native settlements somehow upgrade?

 

[Antilogic]

I think that is a bug, I have never heard of a rule allowing for native villages to upgrade (although I think they can breed their own horses). I've been googling around to try and find a solid rule but haven't found a link.