The RAF is very selective in who it recruits, especially for officer and aircrew.
Primarily it is looking for people who will pass the training with the minimum number of extra training sessions and therefore not pose a training risk which associated costs.
Having been part of the training system, albeit a few decades ago, the thought of only 10% of those selected finally graduating whould have lead to a major overhaul of the selection process. Such an ubsubstantiated figure is highly improbable.