Whether you're legally blind or not, legally deaf or not is binary, but each person is on a spectrum and when interacting with someone we might want to adjust to their level where it matters. You don't talk the same way to an elderly with only half of their hearing left, even if legally they're not deaf. Or choose your color schemes with color blind people in mind, even if they won't be registered for governement assistance.

That's where I see the distinction between the administrative part of it, which needs binary criteria because of the management nightmare it would be to have gradual scales for everything, and the human/everyday life side of things, where caring about the different shades matters a lot IMHO.