Theres a middle man between the assessors and the decision maker which are the auditors.
There are two levels of auditors, one which is within ATOS or Capita, they are usually the ones who tell you to deduct points (and on some very rare occasions, tell you to add points) as they pick apart your assessment for any inconsistencies, e.g the assessor saying the person can't engage with people yet the auditor may notice in a peice of medical evidence that a teacher has wrote in a special educational needs review something like "is mixing well with a couple of peers in class", therefore then asks for it to be deducted due to this. Or that you have scored someone for needing to use aids to cook due to grip yet they notice in your social history that they have a hobby of knitting or something, therefore ask you to remove points as that hobby would contradict having poor grip.
These auditors don't audit every assessment, but it is a very high number. Every assessment which scores enhanced gets audited.
Theres also another layer of auditors at the DWP who do a similar thing though I know less about this, however they only pick out a very small number of assessments to audit like 1 in a 1000 or something. I think this is more to do with getting performance stats. This can get bounced back and made to change too.
The decision maker from what I was told rarely disagrees with the final assessment.
I agree with Michelle, the assessors I worked with were all very caring people who tried to be fair there were only a couple who's judgment I would question. Most people didnt stick it out more than a few months (some didnt even last a week). There were a couple long timers there who hated the job with a passion however they just couldn't take the pay cut of going back to NHS and the hours worked better around family life therefore just sucked it up and got on with it.
I did however work with some senior staff both clinical and non clinical who's ethics I would question.