its cuz your front camera shows the view of you that everyone else sees and not what you see in the mirror.
What i mean is, in the mirror you see the opposite of you basically, and you're used to that. So, whenever you see what you look like in a front camera picture or somehow from someone else's point of view, you'd look weird, when in all actuality, people usually think that that is the you that looks better because they are also used to that you.