Why do people like to pretend they know things?

So, I was talking to a friend of friend today that I knew from about four or five years ago; we randomly bumped into each other and recognized each other, and started talking. Eventually she asked if I had served a mission and I said that I had and that I served in Japan. She asked me what kind of Japanese I spoke and I know that there are many different dialects, but most people don't know that. So, I was somewhat surprised at her asking me that and asked, "Do you know the different dialects?" and then said, "I speak Tohoku ben and Hyojungo and a little bit of Sugaru ben." She told me that she only knows of Regular and Mandarin Japanese. I told her that there is no such thing as Mandarin Japanese, but there is such a thing as Mandarin Chinese and that was probably what she was thinking. She then got a little flustered and said, "My friend speaks Mandarin Japanese. Are you sure there's no such thing? I know that's what he speaks." Now, I consider myself, having a Japanese minor, to be somewhat knowledgable on the subject and she knew this because she had asked if I were studying Japanese earlier in the conversation. How could she have the gumption to second guess me on something so commonly known? What's so hard about saying, "Oh, sorry, I must be mistaken," or "Oh, I was confused?" I wouldn't have thought she was a bad person, and I wouldn't have thought any less of her. Whatever, I just hope she feels like she's made herself look better by trying to convince me that there IS such a thing as Mandarin Japanese.