See my comments above. WL candidates will be offered slots if the number of appointment turndowns (whenever they are received and whatever the reason) exceeds the number of expected acceptances. It's not as if USNA offers 1250 appointments for 1250 slots. They offer some higher number b/c they know there will be turndowns -- to attend NROTC, another SA, a civilian school, etc.).
That may be why USNA waited so long last year to notify WL candidates -- I can't say.
However, it is EXTREMELY rare for someone to be offered an appointment at the very last minute -- such as the day before I-Day. As noted, if someone doesn't show up, the slot goes unfilled. It is very sad in that the slot didn't go to someone who would eagerly have showed, but these cases are very rare.