Indeed straight As wasn’t even a guarantee to happen every year but 50% had SATs >1500/1600. We had many kids getting 5 on the AP but B for the course (our AP Chem class actually had exclusively 5s with 1 or 2 kids getting Cs). Our junior and senior english classes were taught almost exclusively by former college lit professors and our syllabi and grading reflected it. In all of high school I only got an A- on two papers - in all my literature classes at Brown I never got less than an A. We’re not really any smarter, we’re just much better taught and much better prepared. It’s hard for the schools to resist such known commodities - especially when the connections between the guidance counselors and the unis are such that SCEA was treated as ED by our GC so the school knew that student was enrolling if accepted.
Depending on what your definition of “a good number” is, a lot of people overestimate the number of legacy admits coming out of these schools. Rich, well connected kids who get unpaid summer internships that sound impressive instead of working at the local super market and get private tutoring for a year for the SAT- absolutely, but of the ~45% of kids (~20 kids) who went to ivies alone (not even doing the ivy+ thing here) when I graduated in 2005, only 2 were legacies (1 Harvard, 1 Penn). Even if more of us were legacies somewhere, we usually went elsewhere so unless you think the legacy status has cross over, it’s not a factor. If 10% legacy is “a good number” than my apologies, you are spot on - I would assume you meant more than that though.
Where it’s really huge is these are the URMs (who made up ~25% of the class) that schools fall over themselves trying to recruit - particularly the ones who come through Prep For Prep (https://www.prepforprep.org/page). Those kids are pretty much guaranteed to be successful with all the support they receive between Prep and the high school.