Originally Posted by
dave_261
Specific case... flying LHR-ORD. There are 4 flights the day I'm flying: 3 have zero C availability, and 1 flight has 4 C-seats available (but not the flight I'm on).
I'm guessing there's an algorithm as to why that one specific flight has upgrade availability.
Secret algorithm: the empty flight has availability.
Typically the daytime flight (leaves ORD in the morning, gets to LHR late at night) is less popular than the other (depart afternoon / arrive morning) flights.