For whatever is worth, I just got a first-hand information about the actual admission rates at our local BS, which appears on a few top-30 lists on Boardingschoolreview.com. Without revealing too much, I’ll just say that the actual admission rate is 1.5 times higher than the published one. Conversely, the actual mean SSAT score is 10-15 points lower than the published one. Straight from the horse’s mouth. I don’t know about other schools, and make no claims about their stats.
An older discussion on a similar topic is available here: http://talk.collegeconfidential.com/prep-school-admissions/1470922-admissions-stats.html.