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, without needing flights to be full.
On top of that, you have uneven demand causing completely full flights, and while it doesn't add 6 to the entire average it adds a notable fraction of 6. Note that nobody actually claimed they added a full 6 more passengers.
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.
80% of 100 is 80 80% of 106 is 84ish