Why? I think (I didn't analyze anything) that is because I'm just being myself. I talk to the interviewer like it's a friend. If I think of a joke I just say it. Thoughts like "will be it ok if", "should I say this or that", etc NEVER cross my mind. I'm having a conversation, not a test. Same for technical stuff, if I don't know something I just say I don't know. Hopefully I still have an idea, so I would say something like "I don't know this / never used that, but my first guess would be X", then usually it ends up being a discussion that at the very last would teach me something, even if I don't get the job.
That being said, I understand it's hard to do stuff like that if this is not your default mindset and you are being close to desperate.