How do you cast more ballots in a county than voters registered?

Some folks have done an analysis of just one Ohio county's votes. What they found was that in numerous precincts the number of ballots cast was MORE THAN the number of voters registered. Which raises an obvious question. I've heard that In MN you can just walk in and register on the spot on election day. Anyone know how it works in Ohio? Allegedly, there were some 97,489 MORE votes cast in one county alone than those who were registered.

Anyone have the answer to whether this can occur legally?