I don't know where you're getting your information but neither Forbes or Wikipedia list any county in Michigan in the top ten. As a matter of fact, the closest is Livingston County ranked as #95, a far cry from top ten.
"Oakland County is a county in the U.S. state of Michigan. The 2010 Census recorded a population of 1,202,362. (...) It is among the ten highest income counties in the United States with populations over one million people..."
Maybe "over one million people" is the difference in our information. Fifteen years ago or so, Oakland County was said to be top three in the nation. I'm not sure if that was earnings, wealth, or what...heard that on the local radio station.
Regardless, I wonder where your perception of Michigan comes from? Saw Detroit on the news, I bet.
Oakland County is in the top ten for populations over one million. Pontiac drags it down. Livingston County doesn’t have Bloomfield money, but doesn’t have any Pontiacs either.