Why do we appear as a mirror image of ourselves on FaceTime?

I find it annoying that we appear as a mirror image of ourselves on FaceTime.

When the camera is facing us, it should see us as another person would with text in it's proper order, not horizontally

transposed.

In Photo Booth, the same thing happens, but when I take a selfie (not out of vanity, I assure you) the result is a transposed image that comes out the correct way round.

On this basis, does the person I'm speaking to see a correct image of me, or is his/her vision of me the correct way round?

I have been watching a video of a friend recording a live video of himself sending out a message to ex soldiers. His image was transposed because his regimental badge (embroidered on his shirt) was horizontally transposed. I would have expected to see that badge the correct way round.

In Preview, I can transpose an image so that text shows the correct way round, using Tools> Transpose Horizontally.