It was only the RAF that informed me, years later in "preparing" for a side career in selection, that candidates who were "unprepared" for the selection tests and succeeded , were later proved to be a safer bet for success in training.
My third attempt for RAF selection saw me "prepare" by practicing on Brighton pier for the co-ordination tests. For mechanics I read "how Cars Work". A C Comode's "Flight without formulae" was a delight. Quick chat about the Leadership tests with a neighbour up the road (retired RAF Group Captain) where he informed me to just make sure I was noticed. "Get out in front in every group discussion and exercise" were words ringing in my ears as I swung on the end of a rope barking out orders all over the place to everyone. I was offered a place at RAF Cranwell.
It is one thing to "prepare" and another to become so informed as to whizz through selection tests because you are "practiced" and "rehearsed"., thus giving the Selection Board a likely false impression, risking their training investment.
For intense and competitive selection , it helps if you know someone.