Many of the kids in selective colleges learned to grow up being perfectionists and highly driven and thus sensitively cognizant of “the bar,” whatever that may be. They tend to constantly measure themselves against others or the conceptual figment of their imagination. In their habitual measuring, therefore, the imposter syndrome is born because they were conditioned to measure themselves against the higher bar and the accompanying feeling of not measuring up to that bar. They’re often highly accomplished kids, yet with surprising poor self-esteem. One can point to all of their accomplishments, the envy of so many, but these accomplishments mean nothing to them, because they’ve been conditioned to always look up, not within.
The imposter syndrome isn’t about those kids who were somehow “lucky” in the admissions. It’s also felt among those in the higher rung. These kids have typically earned admissions to several selective schools, not just one due to randomness of luck.