I think I get it. The city screen rounds down for displaying individual buildings, but retains the fractions and adds them together to calculate the total happiness.
This is what I was looking at:
View attachment 463964
Ulundi gets +2 happiness from the difficulty level (Noble) and +1 happiness from Palace. With Culture spending at 30%, the left panel says Arena gives +3 happiness (+2 innate, +1 from cultural spending) and Theatre gives +2 (+1 innate, +1 from cultural spending).
But that isn't exactly true. The top panel says 12 total happiness. I can see these sources of happiness:
- 2 from difficulty level
- 1 from Palace
- 3 from Cultural spending at 1/10%
- 3 from Arena
- 2 from Theatre
and that adds up to 11, so the 12th must come from those fractions being retained and added. I guess this doesn't come in BTS because the only building with less than 1/10% is the Colosseum (and its UBs), and you can only adjust culture spending in 10% increments, so you would never have fractions adding up that way.
I don't know if I would call this a bug, but it does result in unexpected behavior based on where you're looking.