I can't answer for SLS but I know about NASS as my son attended it last year. Any parents that attended the ending ceremony of NASS, were briefed on statistics for the Naval Academy and NASS attendance.
NASS just started taking 750 candidates per session as of last year, NASS 2008, for a total of three sessions or 2,250 total attendees. The year prior, only 600 per session attended and that's where the approximate stat of roughly 50% of attendees, receive offers of Appointment to the USNA. That number would be about 900 +/- for a total number of 1600 +/- Appointments offered.
Again I can't quote the SLS facts but, I'm sure they required PSAT, SAT or ACT scores, and if the application is anything like NASS, they use it for your preliminary application so they know your resume. That's why they call you a leading candidate. IMVHO