There's a discussion going on on the Health Unlocked NRAS site about this.

Basically someone has suggested that the government's rules are not that draconian because there has to be repeatability and consistency in the ability to walk 20m. Hence is you can just make 21m then have a rest and can only then do another 15 m you will get the higher rate. Likewise it recognises that there may be good days and bad and the measure has to take account of that. How ATOS do that, of course, is another question

Once again thanks Paul you might find what I have just put on the topic quite a good idea take a look.