As I get into the last few days before our trip, please help me to understand how the really popular FastPass+ attractions (Meet Anna and Elsa, Seven Dwarfs Mine Train) become available. I've managed to acquire individual FastPass+ selections for those attractions for our party, and have been repeatedly trying Select New Time, which has slooooowly and occasionally allowed me to move those selections toward overlapping times. What I don't understand is the most common reasons that people cancel their popular FastPass+ selections.
By way of contrast, consider really popular Advance Dining Reservations, such as Be Our Guest. Well, it's free to make Advance Dining Reservations, so people make a lot of them -- for example, if they are planners trying to deal with other people in their party who have a harder time committing to a plan (ahem). But there are penalties for canceling Advance Dining Reservations too late, so people cancel them so as not to get charged. Alternatively, people might decide to eat that meal somewhere else in that park, and so cancel those reservations; maybe they're going to a different park that day; or maybe they're cancelling their whole trip.
Now return to FastPass+ selections. You only get three a day, so you can't really "hoard" them. There's no cost for missing them, though, so there's little incentive to cancel. Of course, people who make "practice" reservations cancel them (I hope!) out of the goodness of their heart. But that doesn't seem to apply to a lot of people, especially a few days out.
People could decide to cancel FastPass+ selections in favor of other attractions, but it seems unlikely that a lot of people are saying "I really want a FastPass+ for some attraction much less popular, instead of for Meet Anna and Elsa or Seven Dwarfs Mine Train." Sure, maybe they or their kids decided that they had outgrown Anna and Elsa, or that Seven Dwarfs Mine Train was too scary or too boring. But, again, that can't happen too often.
People could decide to go to a different park that day. I guess that happens? Or people could cancel their whole trip, including their tickets, and presumably cancelled tickets free up their associated FastPass+ selections. (Do they?) Yet again, these seem rare.
When park hours change, of course, more FastPass+ selections become available. But that doesn't apply here.
Obviously, people who change the time of their popular FastPass+ selections free up new time selections, but that doesn't change the total number of available popular FastPass+ selections. It's a mystery to me.