>you can just objectively measure if someone is fully blind
No not really. Blindness is a spectrum.
https://www.cnib.ca/en/sight-loss-info/blindness/what-blindn...
https://www.perkins.org/what-blindness-really-looks-like/
That's presumably what the word 'fully' is being employed for.
That's presumably what the word 'fully' is being employed for.