Is it really that high when you consider who they tend to work with?
To the extent I have seen them publish stats, they seem consistent with what I have seen at any relatively selective private HS for which I have seen similar stats. And I think there is some value added, but I think a lot of it is also just that these kids had a lot going for them to begin with in terms of other resources.
And to the extent they are heavily curating activities lists and such, I sometimes wonder if that is even a major source of the actual value added, as opposed to something that parents expect, so they give it to them.