Interviewers are Human Too 

As you prepare for your interviews, you will prepare for the types of questions you might get in an interview. But no matter how much you prepare, you can’t control for everything.

Not matter how much you practice, you might end up with a question that is out of bounds. Or you might be graded more harshly than others for reasons that don't always make sense. 

Studies on Bias

See story on judges being harsher just before lunch. 

Here are some thoughts from Adam Grant: From 2013 and more recently from 2020 about job interviews being broken. 

Complexity of Interview Prep

Because of human bias, I try to prepare you for a number of different scenarios. You may over-practice or under-practice for questions you never get. That is part of the complexity of interviewing.