Does anyone teach the interpretation of statistics at Westminster at all?
A child is not a coin that you toss up in the air and which might equally come down heads or tails. And even with a coin you need a massively large sample for the 50% thing to work out. If you just toss your coin 50 times you can't predict the result with any accuracy: the first 49 may come up heads and only one tails. And that's with tossing the same coin, or a coin of exactly the same weight and measurements.
But with a child, there are so many factors to take into consideration. It is not the case of the same child, or a clone of that child, applying again and again.
Dd attends a college which has a very good reputation for training students for the performing arts and sending them on to HE in this field. The year before last 55% of the students who tried for a well regarded HE school in said subject got in. Looks promising for the students of the next cohort, doesn't it? That year nobody got in. So what do I think of dd's chances? Well, that depends on her, not on the performance of the people around her. It is not a question that can be decided by statistics.
Children are individuals. Apart from giftedness/developments in puberty/attitude/ability to work hard (which is often impossible to predict), the biggest factor that decides whether your child turns out to be that child or not is whether they are the type that will enjoy this particular school and feel at home there.
I had a friend who went to one of the top public schools in the country. Very gifted lad, but hated it so much he spent the rest of his life trying to sabotage his father's educational ambitions by bumming around. Otoh my BIL who went to a poor comprehensive hated that so much that he developed a negative attitude towards education. Dh and I both had school experiences good enough to want to carry on with HE; he went to a private school, I to a comprehensive. It's about finding the right fit, not about statistics.