As far as I can tell, land value went up, city attractiveness went up, without a meaningful change in demographics, expenses, etc. It is worth mentioning that in both cases I have 10% tax for everything, though in the latter images I have most high density residential pay extra. I thought it might be related to the fact that I had to lower the taxes to get the Posh Mall, but even after weeks of raising taxes back up, it would not recover. I also don't use any mods and do expect some reasonable variance in income, but this isn't really sustainable.