I've simplified this a bit but - For a non-medallion flyer there is usually a large number of seats in the first 3rd of the cabin that are reserved for Medallions that appear unavailable on the seat map for non-medallions. These seats open up for general passengers closer to departure. Also as (if) medallions clear for upgrade at 6 days, 4 Days and 2 days pre departure, their seats will also become available. Hope this helps.