Originally Posted by SeattleDavid

They have lots of data about each and every route and flight that tells them what proportion will fail to show up and what number of last minute bookings there might be. This will be heavily influenced by the route and the time of day and the frequency of flights. On routes where there are numerous flights per day I would expect them to be looking at the overall demand, not just the single flight - this clearly happens on routes like LAX-JFK, JFK-LHR and LAX-LHR, where there are multiple AA flights as well as (for LHR) many BA flights.

If you track EF for a few days it becomes clear that availability in all classes fluctuates up and down much more than you would expect and certainly much more than the number of empty seats varies. I've seen flights go from J0 Y7 to J7 Y0 and back to J2 Y2 in the course of less than 24 hours.