4 ms·

Skipping all the details, it's because you have a left eye and a right eye, not an up eye and a down eye.

• RickHull 15y ago
  The effect does not go away if you only have (or use) one eye.
  • david927 15y ago
    Turn the mirror 90 degrees and nothing happens. Turn your head 90 degrees and it flips up and down.