Someone should ask a Michigander about this, but I'm guessing this was because there were two seats open on the supreme court so people got to make two votes. This means there was actually a substantial undervote for the supreme court, which is what I would expect, since a lot of people don't care about it or don't know enough to make a useful choice.
I AM a Michigander. Two seats were open and there were 7 candidates with the top two vote getters winning the election......