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?)

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