overbooking is a company policy.
Each company studies the passenger behaviour on a certain route and establishes a pattern.
For the same route different companies have diffierent no-show policies.
Some years ago on the route PDL-LIS (Ponta Delgada, Azores - Lisboa) TAP Air Portugal had a 'underbooking' policy.
For a 120 seats flight they only confirmed 117 seats. The 3 seats left only became available with supervisor's pemission one hour before the flight, seats were reserved for the transportation of sick people or government between the Azores and Mainland ...