Originally Posted by
pinniped
I'm unfamiliar with this mythical 50% full flight.

That is because if 50% of the flights are at 50% load and 50% of the flights are at 100% load, 67% of the passengers are on full flights even though half the flights are empty. Or a similar calculation.
If 50% of the flights are at 25% load and 50% of the flights are at 100% load, 80% of the passengers are on full flights even though half the flights are empty!