Usually when you think it is the last summer, it is not. Our DS's date changed 4 times, but he went rated.
Sometimes the dates change because their training class date can change. Typically, AFA grads go 1st, than AFROTC, and OCS is placed into the mix.
There are also cadets that are alt. on rated boards,but have a non-rated assignment in hand and are picked up later for a rated slot. That messes with their class lists too. Opening up more spots earlier.
Additionally, like our DS, his 1st date was May 10th...he didn't commission until May 26th. That opened a slot for someone else. His next date was May 30th, but it was a 4 day travel, which kicked him out of that. The 3rd was Aug. 1st, but than they switched it to Sept. 30th. Everytime one part of the puzzle moves, it impacts another.