Airlines routinely sell more tickets than there are seats on the aircraft, and in the great majority of cases nobody is affected. The practice rests on a forecast that is reliable in aggregate and unreliable for any individual flight.
A predictable share of passengers never arrives
On most flights some booked passengers do not travel, having missed a connection, changed plans or bought a flexible ticket they had no firm intention of using.
A departure at full occupancy with unsold seats is a permanent loss, because a seat on a departed flight cannot be sold afterwards at any price.
Selling slightly beyond capacity recovers those seats, and the revenue involved is significant across a network operating many departures each day.
The number is forecast per flight, not per airline
The rate at which passengers fail to appear varies by route, day, time and fare mix, and carriers model it from their own history for that specific departure pattern.
Flights carrying mostly leisure passengers on non-refundable fares show low rates, while flights heavy with flexible business tickets show much higher ones.
The forecast sets how far beyond capacity sales may run, and that limit is adjusted as the departure approaches and actual bookings become known.
Volunteers are sought before anyone is denied
When more passengers appear than seats exist, carriers ask for volunteers to travel later, offering compensation that is raised until enough people accept.
An auction of that kind is cheaper than the alternative, and it allocates the inconvenience to passengers whose plans can absorb it most easily.
Involuntary denial follows published priority rules, and compensation in that case is set by regulation rather than by negotiation. The specific entitlements differ by jurisdiction.
The cost of getting it wrong is not only compensation
A denied passenger must be rebooked, which consumes a seat on a later flight that could otherwise have been sold, and may require accommodation.
Disruption also cascades, since a passenger delayed onto a later service can miss a connection and require handling again at the next point.
Carriers weigh these costs against the revenue from the additional seats, and the balance is why overbooking levels are modest rather than aggressive.
Aircraft changes cause most visible problems
Many denied boarding situations arise not from overselling but from a smaller aircraft being substituted for the scheduled one after a technical fault.
Weight restrictions can have the same effect, since hot conditions or a long sector may limit how much load an aircraft can carry regardless of seat count.
These cases are indistinguishable from overbooking at the gate, which is part of why the practice attracts more blame than the arithmetic alone would suggest.