On any given flight, there will almost always be some people who cancel at the last minute, get delayed on their way to the airport, miss a connection, etc. From the airline's perspective, there's now an empty seat going to waste (unless there are passengers at the airport waiting for a standby seat). Therefore, airlines will often sell a few more tickets than they actually have seats for because, statistically, they expect some no-shows. But sometimes what's statistically probable doesn't actually happen.