In Cities: Skylines, I notice that my income can fluctuate wildly over the course of a few weeks with no inputs from me. I'll be sitting at, say +4000, and a few short weeks later I'm at -2000. I added no new city services and there is no significant population change which would account for the big loss in income. A couple months after that and I could be at +7000. I would expect any changes resulting from population change to be gradual (barring a mass exodus or disaster) and occur over several months, if not longer.
What causes these big changes in the weekly income?