Approximately 60% of American households earn less than $100K. That's quite a difference in relative size.

Households with earners in their 20's and early 30's don't tend to have a lot of children of university age. One would want to use the median income of households with university-aged children.

(Median income by age rises sharply from 20->40, then flatlines... the median age of a mother is around 27?)

I'd also think you'd want to adjust by geography based on COL. A two earner family earning $200K in SF or NYC is different than one earning that in suburban Arkansas.

The population of people who apply to a nice college, or even college at all, is probably not representative of the average american household