So, I've just made a bunch of experiments and noticed something very weird...
Before I took a city, Civil service required 333
When I immediately decided to raze it : Civil service requirements decreased to 325
And permanently so, even after the disappearing of the town.
When I decided to annex the city, then Civil service cost increased to 337 beakers. I also took the culture hit. And all of this was permanent : penalties remained even if I burnt the town (even after its complete destruction) or gifted it to somebody else.
When I puppeted it, Civil service cost increased to 337. However, if I sold/gifted the puppet : -> Civil service cost decreased to 325 !
Where does the decrease come from ?
I guess it deserves a topic.