I saw a "bug" like this the 1st turn my castle was built.
But the amounts were correct the next turn.
I would guess what happens is that garrison income/tax base is computed
before any updates to the civ-levels.
This agrees with my experience of finishing an inn on turn 16.
It was in a city so the civ level should of gone from 1 to 2.
But I could only use 5 days of Work Common Magic in the city even after
the Inn was finished.
But turn 17 I was able to use 10 days of Work Common Magic there.
So in thinking it over the game probably works with the Civ levels as they
were at the start of the turn. Then at the end of the turn it updates all
the Civ levels and then displays them.
Pierre Pero pepper@ecn.purdue.edu