That is not always the case. When I have to present programming technical designs to a room of clients and upper management, I know that it matters. I completely know that it matters. But I research to the point I'm convinced I know more than everyone in the room. Therefore, I go into the meeting confident. And it's nothing to do with, "realizing it doesn't matter."

Convincing yourself it doesn't matter and then having confidence, is much more difficult to do than knowing and understanding each scenario and then having confidence. And that accounts for almost all scenarios. A social situation, presenting in front of clients, dating a girl.

Take them as extremes, using a speech as an example. Let's say all you think about is realizing it doesn't matter. Then you stand up to present and you are convinced it doesn't matter, but you start to realize you forgot to think about the speech. So you don't know what to talk about. Your heart starts racing, and your voice starts cracking, and all you can think about is your nerves.

Now let's say all you think about is the content of the speech. You stand up to give your speech, and realize you know everything there is to know about the topic. So your mind becomes engaged in the topic, and you forget any nerves that you may have had.