It's just that when you look in the mirror, you are looking from the right angle. This is how this thing is explained. It's the same when we are taking pictures. You just need to find the right angle.
Our perception of ourselves in the mirror is always subjective, because it depends on our current state. A person, as it were, "distorts" himself. In anorexia, for example, people see themselves as being fatter than they really are. If I am in a good mood - I look in the mirror and seem more beautiful to myself, if I am in a bad mood and I tend to disrupt my self-image - it seems to me that I look very bad.
But there are also objective things. Have you noticed that the fitting rooms have such mirrors and such wonderful lights that everyone looks bad? Mirrors in shops really distort the reflection. And there are very pragmatic reasons for this. The thing is that we often make purchases under the influence of bad emotions. Marketing is based on this: to create pain and experiences - and then a person will likely buy something, because this will help soften an unpleasant emotion. When we are joyful, we do not need anything, and when we are upset, we begin to “seize” the stress with shopping.
There is only one thing to help here: go to a psychotherapist and treat a violation of self-perception. In general, to a small extent this is typical for us.
The face has asymmetry and we get used to it during our life when we look at ourselves in the mirror. But for others, we look different, because the mirror swaps the left and right sides in places, so we often don't like ourselves in photographs. This has little to do with the question, just an interesting fact.
Of course it depends on the mirror too. But an important factor is generally the mood, as for me. If a person is positive, he may not notice some of his shortcomings, but if you feel so bad, then your brain will not deprive you of suffering about your appearance. Somewhere there is a pimple, somewhere not such cheekbones, a big nose, and so on.
Vertical mirrors pull out the reflection, horizontal ones vice versa.
Much also depends on the amalgam, the layer that is the substrate for the glass. There are silver, aluminum, gilded.
The difference is in the glass itself. Sometimes glass distorts by itself.
Lighting is also very influential.