I have heard from our local NATCA president that the first time you attend the Academy it is pass/fail, and that any and all subsequent classes you attend at the Academy are pass/pass. I was told it had to do with the the fact that the Academy is stage 1 of training and once you've passed it the first time, even if you are sent back to the Academy--say because you are being reassigned to the terminal option as is often the case--you cannot fail because it's still considered stage 1 which you have already passed once.
Honestly that sounds about right - a good example of this is the recent reversal of two PATCO rehires' reinstatement following successful arbitration. They were rehired, failed the PV for whichever course they were in, and were terminated by the agency like a new-hire would be...except for that they're not new-hires, and the arbitrator agreed. So they're getting reinstatement plus back pay.
If they made rtf pass/fail it would only apply to new hires. Any controllers coming from an up/down for rtf wouldn't be pass/fail. Along with controllers coming from a different option like center or tower only. Just new hires going straight to a tracon would be held to the pass/fail setup.