The reality is that most interviewers are not very good at interviewing candidates. This is to be expected since most do not receive any training and questions tend to be ad hoc with interview results frequently determined by gut feel. One unfortunate by-product of this is that many interviews contain difficult questions that may not actually be that important or helpful.
The New York Times recently published a story about regularly asked difficult to answer interview questions. If you ever wondered about how to answer a question like: “What is your greatest weakness?” do take a look at this short article for some perspectives on how to answer.