I believe they understood your question.
Your chance (with 90%+ SSATs) is about 18%.
The advice, because of the reality of a 16-20% chance (with an “excellent” app) is to not focus on one school. 2,500 will apply for 300 seats. 2,000 will be excellent. Everyone has straight A’s, is the leader of a student organization, and excels in the Arts or Athletics.
If you are aiming “high” consider a pool of similar schools such as Andover, Lawrenceville, and St Paul’s. You are one (possibly) of 2,000 highest quality applicants hoping to secure a total of 1,200 freshman seats (estimating) between those 4 schools in my example. All 2,000 are “qualified”. You have to hope you are the puzzle piece they are missing. There will be no rhyme or reason you can understand. Students with 99% SSATs and “perfect” apps will be rejected. 800 fantastic students, the top 0.5% of their middle schools, will be rejected and they and their families will be (understandably) shocked.
So, read the other “chance me for Exeter” threads (so much detail to find), study for your SSATs, and practice your interviews.
As I have said to others, my own son is doing VERY well at one of these schools and waitlisted at two others. So he could have easily gone 0-3 or 3-0 for acceptance.