Now that you mention it, I have kind of noticed this. Social skills appear to consist of a certain amount of glossing over details that you don't want to derail the more important thought processes. You don't mention aloud exactly everything you are thinking, because if you do you end up labeled "dork".

Writing/typing is a little different because you know in advance that whatever comes out is not going to be percieved immediately by anyone, so it goes through fewer filters at first. Afterwards you have the chance to edit it to be less embarassing (flow or emotion) or more correct (factually or logically).

Overall this means you can reveal more without risking as much. Rather than revealing half-formed thoughts (which people find to be threatening), you can fully develop them before revelation, and make yourself sound sophisticated. Even so, some thoughts are best left out (in some contexts) because they distract from the main point -- you can only develop so many concepts in a given amount of space.