When I look into the mirror of a car, whether its the rear view mirror or the mirror on the visors my facial hair, regardless of how much or how little, is very noticeable. However, when I look in my bedroom mirror or the bathroom mirror, I can see the facial hair, but its not anywhere as enhanced as the one out in the car. Also when I get unfortunate and get a zit here and there, in the mirror in my bathroom its not really bad, but the mirror in the car makes it look like its the main focus on my face. Which mirror is more true to what other people see? Because I really don't know which one to trust.
I do agree with the lighting, but don't car mirrors make things look closer than they really are? Unless you're standing incredibly close to the bathroom mirror (in which case, you won't be able to see much of anything because you're so close) things will be more noticeable in the car mirror.
Take the mirror from your room outside and give it a test. If it looks the same as the mirror in teh car you have your answer. Also keep in mind that we are more aware of our flaws than other people - something that's a huge deal to us may go unnoticed by others.
I know what you mean, happens to me too. I agree with the other answers, its mostly about the light. You see, orange lights seem to make everything look better than it should, hides zits redness, hair, wrinkles and smooths face, while the white neon makes your features "darker" and so unfresh and unattractive. The car has natural light (thats if you're out on the road) and so everything is more visible. Also, you can see the difference in where you place the light.