For speed, MySQL doesn't rebuild the tables every time that a minor change (editing posts, deleting posts, pm's, etc); on large boards it would grind to a hault as an optimize would lock the table for the maintenance option to complete.
The "overhead" is simply an indication of how much "dead space" is in the database; an optimization once a month (or whatever your backup schedule is) should be well sufficient to rebuild the tables to clear out the overhead.
