As an interviewer I find it easy to catch candidates who are regurgitating a memorized answer, but catching people who know the answer in-and-out is really hard. I've had the exact same experiences on the interviewing side of the table as well.
I think interviewers tend to overestimate their ability to catch people who have seen the question before and miss on tons of candidates who are good at answering seen questions.
I think interviewers tend to overestimate their ability to catch people who have seen the question before and miss on tons of candidates who are good at answering seen questions.