“Over-under” ballots are those where someone either filled in two or more blanks, or left the question blank entirely. If a manual recount is ordered by the state after the machine recount that is currently under way, those ballots will be reviewed and possibly tabulated.
This is part of the recount, now underway.
Miami-Dade is ahead of Broward because Miami-Dade has newer equipment, bigger budget and staffing and has been running 24 hrs/day.
Broward has been asking for newer equipment and funding for 10 years now but the State of Florida has been ignoring them, so they’re stuck with old, slow equipment. It still works but it’s not as fast as the updated stuff.