My experience has been like yours. I don't prep, I don't memorize, and I don't worry about the answers, because they don't matter. I just put my engineering hat on, work my way through the problems, and communicate as I go. Maybe I find a solution right away, and they throw me a twist to take it further; maybe I struggle, and they have to give me a hint; maybe I work at it for a while and explore a couple different approaches, but never actually get there. So what? The process is the point, and I show them how I work. I almost always get an offer.

I think maybe some people are mistaking job interviews for university exams.