I think these flights are "bulking out", in other words, becoming space restricted. When I was in a previous job like this, but not with FedEx, when you ran out of room, you moved things in priority order. When there wasnt enough room, some of the pallets were simply left behind to wait for the next flight. But if the next flight went in priority order, it could cause a serious back up.
And by priority order, its not necessarily other ipads that were ordered earlier, it is the massive amounts of cargo that get moved back and forth on these planes. Produce and meats, for example, have some of the highest priorities, along with other items that are perishable. These also fetch the highest cargo revenue per pound...another reason they go first. With the stuff going on in Japan, I wouldn't be surprised if there is cargo being circulated through the system to get it in/out of Japan.
Too bad there is not a patience app. 😀