I would expect that if five years in a row the "variable income" happens to always be around X, people will start to expect it, will make this expectation a part of their personal finances, and if the next year the income is only X/2, there will be riots in the streets.

Doesn't this kind of already happen with the stock market? I can imagine rioting in the case where people loose half of their investments overnight, but I think it'd be different for a cut in dividends since any economic downturn that reduces land values (and thus land taxes) would correspondingly reduce rents and thus people probably wouldn't be left immediately destitute if dividends cut in half or something.

"Indeed," snapped the other boy. His stern face lightened somewhat. "Tell me, what House do you think you might be sorted into? I'm bound for Slytherin House, of course, like my father Lucius before me. And for you, I'd guess House Hufflepuff, or possibly House Elf."

Draco Malfoy mentioned it previously :)