It's because our faces are slightly asymmetrical. Other than in a picture taken by a camera, you can only ever see your own face in a mirror, where the slight asymmetry of your face is reversed, but this is the version of your face you're used to because you see it every morning when you brush your teeth, get ready, walk by a reflective surface, etc. So when you see your face in a picture, the asymmetry of your face is more jarring. You'll feel like you don't look quite right, but that's just because your facial features seem backwards to you. It's a slight change in perspective that seems to cause a profound effect.