Okay, then why is Brown County, Wisconsin included, but Washtenaw County, Michigan, Ottawa County, Michigan, and Kalamazoo County, Michigan not?

According to the 2010 Census, Brown County had a population of 248,007, with a population density of 467.9 per square mile.

According to the 2010 Census, Washtenaw County had a population of 344,791, with a population density of 488.4 per square mile

According to the 2010 Census, Ottawa County had a population of 263,807, with a population density of 468.6 per square mile.

According to the 2010 Census, Kalamazoo County had a population of 250,331, with a population density of 445.4 per square mile.

If you use more recent population estimates, the differences are even bigger, at least for Washtenaw County and Ottawa County.

Brown County has a 2015 estimated population of 258,718, with a population density of 488.1 per square mile.

Washtenaw County has a 2015 estimated population of 357,029, with a population density of 505.7 per square mile.

Ottawa County has a 2015 estimated population of 279,995, with a population density of 497.3 per square mile.

Kalamazoo County has a 2015 estimated population of 258,908, with a population density of 460.7 per square mile.