hahaha . I've thought of this as well, but when in front of a mirror, I suppose you really have no frame of reference, and you inadvertently look better since you're getting active feedback on how you should project yourself. Since we're not in front of a mirror 24/7, i guess we don't receive this feedback and don't make the necessary adjustments needed to look better..
I'm just talking out of my ass, hehe, but this is what I've come to terms with when I noticed it.