I guess it must have been a town hall upgrade... I looked at the messages log and there were a couple of those in there. Why do they usually happen in quick succession (like multiple times in the same month)? I've noticed it with other towns, too, but I guess I just never looked at the city growths.
(Also, if growth is just population delta, then how is it not a whole number? Shouldn't population always be a whole number, and always increase by a whole number?)