It is actually surprising how often this happens ... especially last minute, as I have had the same sort of past success. I was given several reasons that this happens:
1) Most often, as the best seats are now sold as "premium seating", with the venue legally charging 1.5 to 3 times face value, depending on demand, these seats are sometimes released close to the event date.
2) These can be returned promo tickets
3) Eventhough the ticket agents say no returns or refunds, occasionally they will still accept returns (especially on premium seats.)
Congrats and enjoy