Even people without PTSD will use the "you don't know me at all" line. Think about it from your own side - when would you say that to someone? I know I've said/felt this when there have been situations with people close to me whom I expected to understand my perspective, and they did not.
The people we come to know and love - friends, family, close associates and co-workers - should come to understand our character and then be able to, sometimes, predict our responses or desires, sometimes even better than we ourselves can. For example - I know I would feel/say this if someone assumed I was trying to get money from them if I told them about my financial struggles, because that just isn't me. This sometimes happens with my mother, as she always tries to fix everyone's problems. I have a very hard time talking to her about any problems I'm having - financial or otherwise - because she then assumes I want something out of her, which I don't. I just want someone who will listen and/or give advice to help me solve it myself so I can build my confidence and independence.