Kate
An omission is not really any different from a discrepancy. It's still of the form "I said" vs "you said".
If you have provided evidence that you are unable to mobilse 50m and there are no references to this in the reports, then you are entitled to challenge the ATOS "doctor" and the DWP Decision Maker as to why they have ignored this evidence.
Good luck
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems