No matter how much work I put in, I could never get an A in a-level PE; I'm autistic and PE is one of the things I am horrendously bad at. Likewise, I would imagine that there are some severely dyslexic people who could never get an A in English and people with dyscalculia who could never get an A in maths.
With non-disabled students and typical 'academic' subjects, yes, but I do think there are some people who could never get an A in some things due to disabilities, and I also think that subjects like art would be very hard to get an A in if you had no natural talent whatsoever.