I was told by a gate agent one day during an airport appreciation break that often times when there seems to be plenty of seats even on the airport standby list (the 2nd list) that people who purchase tickets online without confirmed seats (like Priceline) will appear and take some of those seats.
It's happened to me several times trying to go JFK-LAS that both the main page and the airport standby page show several seats available, but the closer it got to boarding the number of open seats declined rapidly.