I found an excellent example of something I tried to explain to
underwatercolor a few months back. Most people think I'm a very quiet person, when in fact that's generally not the case. I just don't speak when I don't feel I have something to add to the conversation. Most of the time, my knowledge base does not match with those around me, and that which I can speak to in an intelligent manner does not come up in casual conversation.
Case and point: Womble decided that he's going to try to make soap from bacon grease and wood lye, and asked my advice. I launched into a 5 minute lecture on how exactly to go about doing that, how to vary the ratio to change the texture of the final product, the different types of soap that different fats make, the difference between using potassium hydroxide and sodium hydroxide, etc. The girl sitting next to me was completely taken aback, and asked how the hell I knew that. I shrugged.
How often does that type of stuff come up in casual conversation? How often will someone ask you what the best tomato variety is for their climate, or what's the best wood splitting technique, or how apple genetics work (it's very fascinating I assure you...).
All my friends are tech nerds, and every man I've ever dated has been a computer programmer. I have very little to contribute to the sorts of nerdy conversations they're likely to get into. I've been around it long enough that I can understand the lingo, but the converse is not true. People rarely have any interest in the sorts of things I'm likely to rant about. Thus, at just about any social gathering, I sit in the corner and people watch. All the while, gathering a reputation as the quiet girl that one must be careful not to scare off or offend.