If the nobility consumed a disproportionate amount of goods and services, but were less likely to die from the plague, you could see the relative amount of labor per consumer drop.
They could also be trading with areas less effected by the plague. If your village gets 50% wiped out by the plague, but you sell crops to a city, you are still going to want 100% of the crops collected.
They could also be trading with areas less effected by the plague. If your village gets 50% wiped out by the plague, but you sell crops to a city, you are still going to want 100% of the crops collected.