Oh another trick question!
One of the most important descriptors for mobility is, of course, distance in metres that someone can walk safely, reliably and repeatably, and on the majority of days.
This is something many people can easily work out, because your DH may know he can do eg the walk from bathroom to front door, but not the walk from front door to two houses down.
In order to get around this, the DWP/ATOS always add a trick question about "how many minutes can you walk for?" Minutes-walked is not a descriptor.
The DWP then uses its own conversion rate (IIRC it's something like "normal pace", "slow pace", very slow pace") to convert this time to metres. This usually gives a very different answer from distance on the ground!
This is because:
(a) The DWP conversion is necessarily crude;
(b) Walking 20 m isn't something most people know a timing for. It's not like knowing it takes you eg 15 mins to walk a kilometre. A natural answer is likely to be vague and rounded up or down, or phrased as "a couple of minutes" when you mean about 1 min 10 secs. The assessor can take advantage (I've had an assessor offer me the answer, "I walk around the neighbourhood for ten minutes", when I said I tried to walk 30 m on the pavement as exercise on a good day. Er, no.)
(c) The more disabled someone is, and the slower they walk, the more minutes it will take them to do a very short distance. So the more metres the DWP will claim they must be walking.
In my experience, the DWP prize this fake, falsely calculated distance, and will cherry-pick it over everything you've submitted (pics, Streetview, whatever) that supports the real-world distance in metres, which is the actual descriptor.
The only way round this is to very accurately assess in seconds (not minutes!) the distances your DH can actually walk safely, reliably and repeatedly. Stick to these like glue, do not be talked out of it, and be clear about what sort of pace this is at.
Also, your DH should do his safe, repeatable pace on the long, observed journey to the assessment room. Do not let them hurry him or make him feel impolite. Do take as much time as he would take if there were no one there, and also as much time as if he needed to do the journey repeatedly