Yup, I have it (also called Resting b***h Face elsewhere on the internet). I figure it's a combination of focusing more on what I'm thinking than on what my face is doing, plus human faces not being socially magnetic for me. "Normal" human expression tends to be, as far as I can tell, based significantly on micro-reactions to other people, so there's a constant subliminal two-way communication going on beneath the surface. When someone doesn't automatically engage via that channel, it gives the impression that their expression is 'dead', or that they're potentially dangerous because the viewer isn't getting a stream of constant subliminal reassurances.
It's related to the Uncanny Valley, where people get mildly freaked at things like hyper-realistic dolls and CGI because there's no strongly-connected mind behind the facial expressions, even if there's a puppeteer or facial expression scripter. It's also why some of the better CGI characters are either visibly not 100% human-realistic, or have facial expressions mapped directly from professional actors in order to get those tiny muscle movements right.
For me, it's actually been advantageous on occasion - I can sometimes shut people down entirely by gazing straight through them, like I'm seeing things written on the inside of their skull and considering the best way to disassemble them for a better look. As mentioned earlier in the thread, it's quite easy to give off a serial-killer vibe, or at least the impression of someone who doesn't consider other people to be human. It's honestly been a useful, if rarely used, tool when I've done customer service, or when dealing with minor-league bullies and asswipes in the workplace.
Knowing I have it by default also reminds me when it's time to do human interaction things. "Oh, right, have to do the face thing. Engaging real-time control of facial muscles; spinning up an emotional personality mask; attaching pretending-to-give-a-shit-about-other-people's-lives mental prosthetic; here we go."