Airlines routinely sell more tickets than there are seats on the aircraft. The practice is deliberate, statistical, and aimed at a problem created by the way flexible tickets work.
The no-show problem comes first
A proportion of booked passengers never appear. Some rebook, some miss connections, and some hold fully changeable tickets that let them abandon a flight at no cost.
Those seats cannot be resold once the door closes. An aircraft that departs with empty rows and a waiting list has wasted inventory that had value minutes earlier.
Selling a small surplus recovers that value. The airline is betting that the historical no-show pattern for the route will repeat closely enough on the day.
Forecasting is route and fare specific
The size of the surplus is not uniform. A commuter route full of changeable business fares behaves quite differently from a holiday charter where everyone paid in advance.
Systems model each departure against its own history, adjusting for day of week, season, connecting traffic and the mix of fare types actually sold.
On routes where almost nobody fails to show, the airline sells to capacity and no more. The surplus exists only where the data supports it.
The gate becomes a small auction
When more passengers present than there are seats, staff first ask for volunteers to travel later in exchange for compensation, and the offer rises until someone accepts.
This is a genuine market. Travellers with flexible plans trade their seat for value, and the airline pays only what it takes to clear the imbalance.
Only if volunteering fails does the airline deny boarding involuntarily, and the criteria for choosing who is affected are set out in the carrier's published conditions of carriage.
Passenger protections change the arithmetic
Regulators in many places require compensation and re-routing when a passenger is denied boarding against their will, and the amounts and triggers vary considerably by jurisdiction.
Those rules put a price on getting the forecast wrong, which is precisely their purpose. An airline that oversells aggressively pays for the shortfall in cash and in reputation.
Because entitlements differ by country and by the route flown, a traveller who is affected needs to check the rules that apply to their specific journey rather than assume a general standard.
Why it is less visible than it used to be
Non-refundable fares now dominate leisure travel, and those passengers show up. Forecasts have grown more accurate as the share of changeable tickets has fallen.
Airlines have also learned that a denied boarding is expensive in goodwill, and several have narrowed the surplus they are willing to sell.
The mechanism has not disappeared, but it now operates closer to the margin, which is why most travellers encounter it as an announcement rather than as an outcome.