Comment by rrr_oh_man

It's much simpler than that.

UN urbanization rate is based on the national definition.

The U.S. definition of urban area is based on population (and has been changed recently):

  Under the old criteria, an urbanized area needed to have at least 50,000 residents. An urban cluster was defined as having at least 2,500 people, a threshold that had been around since 1910. Under this definition, almost 81 percent of the U.S. was urban and 19 percent was rural over the past decade.

  Under the new definition, hammered out after the 2020 census, the minimum population required for an area to be considered urban doubled to 5,000 people. Originally, the Census Bureau proposed raising the threshold to 10,000 people but pulled back amid opposition. The new criteria for urban areas shift the urban-rural ratio slightly, to 79.6 percent and 20.4 percent, respectively.

Source: https://www.pbs.org/newshour/politics/u-s-census-bureau-rede... ("U.S. Census Bureau redefines the meaning of urban areas in America", PBS, Dec 29, 2022)