When my friend worked in revenue management, he said that within the near year-long lifecycle of any flight (i.e. from when it's open to book until it flies), some flights on certain routes could be well over double booked, e.g. for Algeria they may at some point have had 350 people ticketed for 150 seats. The algo worked well and he said flights regularly went out full with barely anyone getting offloaded. With combinations of flexible tickets/date changes, no shows, misconnections, etc etc, they were remarkably good at working out how many tickets needed to be sold to maximize space onboard and thus revenue per seat. Fascinating stuff