Interesting and predictable.
Check this out:

The small suburban independent city of Falls Church, Va.—which is treated as the equivalent of a county by the Census Bureau–had a higher median household income in 2012 than any county in the United States, according to data released in December by the Census Bureau.

In the Census Bureau’s ranking of the 30 counties with the highest median household incomes, the City of Falls Church rose from the No. 2 spot in 2011 to the No. 1 spot in 2012, overtaking Loudon County, Va.

While treated as the equivalent of a county by the Census Bureau, the City of Falls Church had a population of only 13,229 in 2012, as estimated by the Census Bureau, and is merely 2.2 square miles in size.