I've never met anyone at Caltech who has not been accepted at one of the top schools. I mean, if you're strong enough to get into Caltech, you're most likely strong enough to get into any of the top 10 school of the nation.
Some students have outstanding academic strengths, but don't get accepted to schools like Princeton, MIT, etc. I know enough examples of this. Caltech admits students almost solely based on technical + academic achievement, and that's not how most schools do it. Sure, someone accepted to Caltech is qualified
to get into other top schools, but that doesn't mean in reality that he/she will get into the other top schools applied to.
So "selectivity" is not solely a function of grades. If it were, the classes of most schools would look a lot, lot different. The fact that several schools foster different kinds of outstanding achievement is enough to guarantee that a Caltech student, though outstanding, isn't guaranteed a free ticket to other top schools at all.