Pardons if this has been covered elsewhere; I didn't find anything with the new Google search though -

I have 15% Unrest in a number of my cities that has persisted long after the usual 30% (from first conquer) wears off.

Some of my cities duly went from 30 to 0% in 5% per turn (?) as usual, but many others went from 30 to 15 and are staying there now dozens of turns after conquering.

I can't make any particular sense out of which have or haven't except that maybe it's due to me having exterminated them? I've exterminated a lot this game BUT can't remember precisely which cities, so I'm not sure if this is the rule.

Anybody else seeing this? Any ideas? I'm playing a Normal/Normal game (Julii).