PIP assessments are often utter bollocks. I work in welfare rights and do in excess of 50 PIP applications a year.
Around half of them are successful, the other half go to mandatory reconsiderations and around 50% of those go to appeal.
When I see the DWP's "bundle", the assessor's report is the first thing I look at. I see the same phrases repeated over and over again. They're obviously a copy and paste job. I've seen several where the applicant appears to have changed sex and back again more than once during the assessment, many where the assessor "observed" something when the assessment was done over the phone, a classic where the client was able to walk in to the assessor's office without using any aids, when they only had one leg and were on crutches, and all sorts of other complete fiction.
The DWP know that the reports are rubbish. It's not uncommon for them to contact the client and offer them an award of PIP in the first few weeks after an appeal has gone in, and they quite often do it very close to the appeal, the evening before in one memorable instance.
My team's success rate at appeals is almost 100% (a former colleague lost one once, around 5 years ago). If the reports weren't rubbish, that wouldn't be possible.
I recently helped someone who lost their PIP at a review. Because we didn't do the application or the review form, we had no idea what they had had points for and they were technically out of time to challenge it, so we helped them make a new application. A few weeks after the DWP got the new form, someone phoned the client and asked them why they were reapplying. They explained, the DWP bod overturned the review decision, increased the daily living amount from standard to enhanced, and backdated it to the date the previous award ended.
The system is completely corrupt imo. I think it's a numbers game: they know that a lot of people won't have the will and/or ability to go through the appeal process and that it's worth the extra hassle of MRs and appeals because it saves money in the long run.