Having "drops" at strategic or random times makes sense.
1. In order for a larger family, say, 6 persons, to see availability of Fastpasses where there was none before, several FPs have to be "dropped" at the same time.
2. If a screen showing FP availability is shown nearly simultaneously to two (or more) guests doing refreshes, it is possible for the second guest to select the same FPs before the first guests hits confirm. So more FPs would be granted than exist. Like, two guests each select 4 FPs after 4 were cancelled and then dropped a second later. This would be more likely to occur with top tier rides such as Flight of Passage and Slinky Dog Dash given the large amount of refreshing guests do for these rides.
So far I have not seen any reference to, or question asked about, selecting FPs and then getting an error message upon hitting confirm.
The system needs the ability to accumulate large numbers of cancelled FPs and drop them all at once to minimize the problems of #2. (Plus process cancelled FPs internally to correct overbookings in the various time slots caused by #2 before dropping any for new guest selections.)