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<p>Because there is a LOT of truth in the claim. The fact is that many high achieving high schoolers apply to a lot of highly selective colleges. And the fact is that many high achieving high schoolers in California apply to Stanford and the top UCs (as well as USC and perhaps Pomona for those that want a LAC experience). Since Stanford is much more selective than Cal, it stands to (logically) reason that Cal will admit many of those that are rejected by Stanford. And fwiw, Cal will also accept many who have been rejected by the Ivies and many other highly selective colleges, such as Williams, Amherst, Caltech, MIT, Vanderbilt, etc… </p>
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<p>What do these have to do with undergraduate admissions?</p>