Another reason you can get tickets later: when a ticketing sale starts a lot of people have multiple windows open and multiple tickets in multiple "shopping carts", even though they may only want, say, 2. The tickets have been taken from inventory and are sitting in the shopping cart even though the person is not intending to purchase all of them (or can't, because the system is getting hit hard).
All ticketing systems have an automatic function where after "X" number of minutes (maybe 10, maybe 15), if the transaction is not complete, the tickets will be returned to inventory for sale.
So, it takes the computer system those 10 or 15 minutes to go through and empty all those shopping carts that are holding tickets but not completing the transaction.