This would be likely because many factions east of Greece can change and reform to Eastern Imperial culture.
The change from Eastern Tribal States to Eastern Imperial should represent a change from a confederation, alliance, or patchwork kingdom of localized cultures to a more broadly controlled state, with a "high culture" that can be identified as the "culture of that state". If that makes sense.
If you look at the Reform Requirements
here, the script should have Pontos' culture change to Eastern Imperial after you:
- Conquer Sinope, Trapezous, Kutatisi, Mazaka, Pantikapaion and Chersonesos
- Win a total of at least 11 battles against any of these factions (spread out between them, not 11 each): Arche Seleukia, Ptolemaioi, Pergamon and/or Bosporan Kingdom
So, as you may have noticed, some buildings will raise certain cultures.
It's all timing. I'd start fighting and conquering now so that you will be able to reform cleanly in the future. Unless you don't want to reform.