Well if you want to know, it's more complex then you think.
Your eyes and brain are playing a nice little trick on you in the mirror, plus the type of lens you are using on a camera will also have a positive or devastating effect on what you get in a photo.
Mirrors:
The human brain actually has a natural "auto fix" feature which corrects the small imperfections in your face. What this means is that if one eye is off just two centimeters, the brain will make your eyes look even in the mirror. It also takes into account depth of features, this goes for everyone you met face to face. So don't think your own brain is just doing it to your own face' reflection. It does this all the time with people you meet face to face in public. In other words we are actually auto correcting faces without knowing it. Now since depth is a important part of the process, you are removing it in a mirror and thus your eyes and brain correct with limited data. This mirror image is what we get used to as we use mirrors a lot, the problem is its incorrect to what you look actually like in reality.
Camera:
When you take a camera image, the camera doesn't have such an "auto fix" like our brains do. It actually only shows what exists which means a flat image but it does try to mimic depth of the subject. Cameras are not a perfect solution to actually get your real image of your face, but they are a good approximation. The weaknesses of cameras are certain things in the photo can affected it even more then you may think about. A wrong lens can effect what the camera is actually seeing, the height of the camera lens can also do this as well. Lighting is also important as it adds the depth which helps define a person's features. If either of these things are off you're going to end up with more of a crooked mouth, eyes e.t.c. Selfies are horrible period for this because they are usually shot very close or at strange angles which make a person look off.
A lot more info is on the web on the scientific part of all this, but this should give you a run down to help you understand whats going on.