There may be more tickets released by the Event Organizer as the event date approaches. Sometimes for popular events, seats with obstructed views may be released for sale.
It may also be a situation when fans find available tickets during the booking process and those tickets are actually on hold while the fan considers the purchase. During that period, which lasts for only a few minutes, no one else can view or purchase those tickets. If the fan decides not to buy the tickets, these tickets will be cleared out of the shopping cart and put back into the available seats inventory, where they become available to other potential purchasers.