Maricopa County ( ) is a county in the south-central part of the U.S. state of Arizona. As of the 2010 census, its population was 3,817,117, making it the state's most populous county, and the fourth-most populous in the United States. It is more populous than 23 states. The county seat is Phoenix, the state capital and fifth-most populous city in the country.
The state capital is in the top 10 most populous cities in the united states
It's the fifth most populous, therefor making it in the top 10. I think the model got confused because it didnt know what exactly a top 10 was.