Earlier quoted context omitted.
But if they could get more people on board, they wouldn't be 80% full. Or are you referring to the times they're completely full, which would go up by 6 seats? Then 6 doesn't get added to the entire average.
It's probabilistic. You can't just sell 100% of tickets ahead of time and be done. Some people don't show up. Some people buy tickets at the last second. The more tickets you sell the more you risk running out of space and having angry customers that need to be compensated. Even if you assume every flight has the same demand, having more seats lets you sell more tickets which lets you get more people on board, withou…
Josefresco above claimed that the 80% rate could be 80% of the new seating arrangement, which just adds +6 across the board. That's what I "took issue" with.