I have it on very good authority that the % who get their choice is greater than 75%, in fact closer to 85%. I am quite confident from what I have seen that the right people go to the right jobs, although you must accept that the Queen has the majority vote when it comes to allocation as the system decides were 'needs are'. In short, if there are 10 vacancies on the Nimrod and 10 students to choose from, the ALM wannabies will not get a look in, that is the lottery associated with 21st Century NCA.
The system is quite fair at handing out carrots & sticks to, and hard work is likely to play dividends. Everbody starts off at the same standard, but not everybody is equal after trade training, socially or professionally. There is no simple: Stars get....... Knobs get.......for both have strengths and weakness.
There is no point in trying to second guess the system.
There are differant qualities demanded by the various trades. The subsequent training systems can cope with certain weakness', whilst being intollerant of others. A long time ago, an NCA who I flew with whilst employed in my previous trade simplified it all by saying "You either have it or you dont". It was not what I expected and did not help my preparation for OASC an iota, but nowadays I can see that he has a fair point.
Bottom line, the training system is relatively short, work your butt off, show that you can fit into a multi crew environment without p155ing everybody else off (not every OASC graduate can do this) and keep your whinges to yourself, you may get lucky. Oh and remember this, it takes most trades at least a decade to get 3 stripes, do not take yours for granted or belittle them by acting like a dick after a pint of shandy.