You've heard my analogy of the wide staircase with multiple people on each stair, right? Sometimes someone has a less than stellar interview (however not so bad that it is an automatic reject) but the grades, scores, ECs, essays, etc are good enough to keep them high enough on the staircase to garner an offer. Sometimes it is the interview that is okay but not excellent in an otherwise excellent applicant while someone else had an equally good application but an exceptional interview showing creativity, curiosity, critical thinking skills, empathy, etc. If we can take 40% of those who are interviewed the top 35% sort themselves out easily and it is the next 10% where the hard decisions are made. At some schools it might be a more strict, "by the numbers" situation where the application and interview are scored in some manner and the cuts are made sharply with little discussion of the applicants at the edges.