There's an article from the Yale student newspaper where the faculty mention that they're looking for students who aren't "cooked" yet. They like to see potential, which would explain why they might admit someone whose portfolio isn't "better" than someone else's--although I'm not sure if classifying such a portfolio as "worse" is really that helpful.
The students at RISD confirmed the same. They're not selecting for the people with the best and most-polished portfolios, they're selecting for people with stellar portfolios who have diverse interests, an openness to critique and change, and who will contribute to a unique dynamic that the cohort otherwise wouldn't have. Though it was said that if you're selected for interview, you definitely have what it takes to study there. If you didn't make it past that stage, it might just not be your year.