I looked up Michael Brown's "step father" on Missouri's criminal database. I don't think he's ever served more than a six-year sentence for any one thing.
Did he serve a full six years or get a six year sentence but served less time than that? Depending on the state you can have a ten year sentence for example but only serve half that time if you behave yourself. In Illinois it is "Class X" felony. I don't know anyone in Missouri so I don't know.