It can be argued that when lots of people use their fastpasses late then the total number of people ahead of you, both virtually and actually, could be greater when you get in the standby line late in the day.
If people all use their fastpasses "on time" the standby wait throughout the day will be X minutes and on average Y people will actually join the standby line each hour. You see, many people decide whether to join the line at all based on the posted wait time at the entrance.
Whereas if lots of people delay using their fastpasses, the standby waits will tend to drop below X minutes and more people in total will actually join the standby line bringing the posted standby wait back to X minutes.
Actually the above really should not concern us. Instead, when I see X minutes on the sign outside the standby line, I should be able to count on it as being reasonably truthful. It would be fitting that, if the wait turns out to be running much longer than posted, I should be able to leave the line and get some credit for the time I waited (for example a fastpass to something else, or granted (adj) re-entry if I needed a bathroom break due to the wait and the park closed before I got back).
Unfortunately a longer than average standby wait for "your last ride" should not be part of the reasoning behind shooting to be back at your resort an hour before your Magical Express bus to the airport. You still need to abandon the line and the ride in order to head for the exit and stay on the plan to be back at your resort that hour in advance.