Thats just the way our brain works. You give it a sample, and it will connect the dots in a way it prefers. You look in the mirror, and see only a small portion of the details, but your brain fills in the rest and you "see" your reflection (but only in your mind's eye
).
When your friends see you, their brains also fill in the blanks, but in different ways, so no two people visually perceive the same things equally.
Pretty neat, eh?