It Is Not What You Think.

It's called a term of art. This means it is not just an ordinary dictionary word, it has a meaning all to itself.

Disability can be defined as the inability to competitively work eight-hours per day, five-days per week (or a comparable schedule). 

If an employee will miss, or is likely to miss, two to three days of work per month due to their impairments, they will not be able to maintain full-time competitive employment. No employer would keep this person around. The same goes if a person is off-task 15% or more of the time due to their impairments.

To be disabled for SSDI and/or SSI purposes does not mean you can't do anything, instead it focuses on your stamina and endurance, relative to the average worker. Legal mumbo jumbo calls this your residual functional capacity, and there are hundreds if not thousands of pages of Rules on what those words mean -- If you hire me, I'll worry about that, so you don't have to.