Only27 I've interviewed alongside a large number of managers. Some of them are good at interviewing. They Know how to set a candidate at ease, they know what questions help candidates to do their best and they treat everyone equally.
Unfortunately there are lots of hopeless, clueless, inept people out there who do just the opposite. It's not you. They are useless interviewers.
I think my particular favourite is the guy who veered off our list of carefully planned questions by chirping up, at the start of the interview "You say on your application that you have good attention to detail, but there are grammatical and spelling mistakes in your application. What have you got to say about that?"
Sometimes they are prejudiced. I once had a boss who was prejudiced against women who did not wear makeup - and people who did not polish their shoes And sometimes they have a favourite candidate lined up.
I remember years ago, when I was working in the health service a friend/colleague went for an interview as head of HR in a hospital. She was the best candidate but she didn't get the job. She felt she'd 'done a bad interview" The woman who did had a background in catering and my friend felt better when it transpired that the successful applicant had been having an affair with the boss of the hospital.
IT'S NOT YOU!
However some people are skilled at being interviewed if you are going to lots of interviews, treat each one as practice at improving your skills at being interviewed.
Look at the Person Specification and think of an example of how you meet each of the criteria
e.g. If one said "able to work on own initiative" think of an example (or make one up) and find an opportunity to talk about it during the interview.
Also try to manage your body language. Turn up looking suitable for the role your applying for. Clean and shiny from top to toe. Shake hands firmly Sit up straight, smile at the interviewers and try to appear energetic and cheerful.
Finally, remember that you don't want to work for an organisation that does not take the trouble to recruit carefully and respectfully. They've got to like you, and you've got to like them. So if they are useless interviewers shrug off the experience because you didn't want to work for them anyway, did you.
Good luck