That was a really thought provoking question, Ronin.
I would guess that from the point of view of the council clerk dealing with the request it came down to walking.
This seems to be a criteria that a lot of non-medical officials use. In simple terms, if you can walk, you are not disabled. If you can only walk a very short distance, you may qualify for a blue Badge - and the form I had to fill in used distances in terms of bus lengths. Now that is a really scientific approach. They gave the distances in metres and bus lengths, when it could have been metres and yards. Trying to be helpfull, no doubt.
At the heart of the problem is that there are too many officials dealing with us, most of whom have no experience, and all they can do is compare the completed form with the list of criteria that they have been told to use. If all councils had to use the EDSS ( Kurtzke Expanded Disability Status Scale ) there would, at least, be some consistency. The down-side is that they would have to get the data from the health service. That could be a real beauraucratic nightmare.
Personally, I like the idea of a mobile ramp. There was a story going round about a large firm of builders who had a set of portable ramps that were put in so that the new houses met the criteria for disabled access to new houses - and which were then moved after the council had inspected the houses and noted compliance.