@belkrax
I thought so too, but maybe when they did the initial retina scanning off-screen, they only did it with their left-eyes?
I did some light research on the subject, here are some semi-useful facts:
-It's based on the blood vessel arrangement of your retina
-Your left and right retinas are not the same
-Barring injury and disease, your eyes are usually the same from birth
-It's more accurate than iris recognition and fingerprints
-For the initial scanning, it requires the person to stare into a device for ~45 seconds
The last one may probably be why they only scanned one eye - probably for convenience and to save time, especially since they are on a time limit.