The relationship between one side of your body - including your face - to the other is not like that of a twin to a twin. Rather, most people don't know that the left and right side are more like sisters than twins. This means that one leg may be a millionth of a degree shorter, or that your left ear is slightly smaller than your right.
Most differences are too small to notice, but facial anomalies are much easier to detect because your face is what people are staring at the most when they look at you. So if you're somehow unhappy with the balance of your face in a photo and want to make both sides of your face more symmetrical, check out this Photoshop tutorial.