An oft-quoted rule-of-thumb is that something like 10% of confirmed reservations simply fail to show up on the day.

As an outsider, you can never know whether a cabin is truly full, anyway.

If, indeed, the term "full" has any real meaning in a dynamic situation in which the number of reservations taken can go well beyond the number of seats on the aircraft, usually with a degree of confidence that that the number of passengers ultimately wishing to travel will not exceed the number of seats; and when check-in (which is when reservations are finally matched to physical space on the aircraft) starts at about -3 days and continues until 45 minutes before departure. Just what definition would you apply to the word "full"?