I can echo some of your numbers but they are a bit different. My son went to NASS 2 2008. He really liked it and is going to apply. He indicated that he was told of all the kids at NASS 40% would end up going to the academy. It was not indicated what was the statistic for the other 60% ie did they apply or not apply.
I heard from two other sources that between 30-40% of the kids that go to NASS decide not to complete applications. They decided USNA was not for them. I met a Marine Officer whose daughter went to NASS 2006 and decided it was not for her. NASS is for those kids too. If its not right for you don't apply go and hope it works out, it usually does not.
Incidentially my son's squad leader ran two squads Session 1 and 2. He told my son of the total 16 kids he was only recommending 2 for admission. None from Session 1. Both my son and the Squad leader were dissappointed in the athletic ability of many of the NASS participents.