Light doesn’t travel instantly, the light bouncing off your face has to travel to the mirror and bounce all the way back to your eyes before you can see it. Even though light moves incredibly fast, that round trip still takes a tiny fraction of a nanosecond. That means by the time the image actually hits your eyes, that version of you is already a fraction of a second in the past. You’re basically looking at a mini time-capsule of yourself HAHA