They aren't perfectly in line, but they're perfect enough, and the black plague wasn't instant, it lasted several decades giving ample time to make adjustments. If you planted food to feed x hundred million people and after the plague you're planting food to feed only 60% of that number, you're going to require a lot less labor to plant and harvest those crops. Repeat for all the other occupations like butchers, tailors, blacksmiths, etc...
> after the plague you're planting food to feed only 60%
All land would used as much it could, and if there was surplus after making food and drink of the best produce, it would become feed for animals. They could grow better things like apples, vegetables, and so on.
