Not from HQ. Just speculating why in the past we can see 20+ people in a school all being promoted in a year, while these days it can be as few as <10.

What I understand (again, not verified because insider sources will obviously not discuss this at length) is, HR will flag officers for promotion at intervals. The algorithm that determines this interval is not known to us, all we know is that it is somewhat related to CEP. Based on this, the school then decides whether to support/not support the promotion. HR will then execute the final decision.

That's why they say that "the rate at which you MAY be promoted depends on CEP, but whether you are actually promoted at each instance depends on your performance". You would have heard this line while in NIE before, or even from colleagues/mentors.