When customers find available tickets either online or through any of the other access points, those tickets are actually reserved while the customer considers the purchase. During that period, which lasts for only a few minutes, no one else can view or purchase those tickets. If the customer decides not to buy the tickets, they are "released" and become available to other purchasers. So if a customer is looking at 10th row centre seats and decides not to buy, those seats become available for another customer.
A venue may release more tickets for Ticketmaster to sell as the event date approaches. This happens for several reasons such as 1) The artist or team may no longer need the tickets that were held when the event originally went on sale, or 2) Once the stage or event area is set up, the venue has a better idea about which seats are obstructed. Sometimes events are so popular that even seats with obstructed views are released for sale!