Yup because 1) a good interview isn’t enough and other factors weigh in as well and 2) people aren’t the best at judging their own interview performances and what the AOs are looking for in those interviews.
Some people are so positive about themselves they think every thing they do smells like roses. 
Others are harsh critics of their own performance.
You see it all the time here on CC especially if you look at the college threads. Everybody comments on how great their essays are. Not everyone can be writing great essays.
Plus, it’s not so much how you say things - polished or not - but WHAT one says in an interview that matters.