@ZandathePanda That made me laugh!
I have an old uni friend who is a team leader for AQA History and AQA politics. He will also be a reviewer for review of marking. These are his insights:
Because there is a tolerance level then a marker giving 15/20, 16/20 or 17/20 is seen as ok. This can bite those on the border of two grades but someone has to be on the border.
However, markers get approx 5-10 mins to mark a two hour script (within pay rates). For those who have 'internalised the standard' and/or are prepared to stand back from something a bit more novel and take more time then usually fine. But, if markers are rushed or less experienced and afraid of the seeding they do to check they are within the tolerance level, they tend to squeeze at the lowest mark. While marking schemes are reviewed and updated during standardisation, and while team leaders can support markers who are unsure, some markers don't ask for help and stick to the lowest tolerance level. Also, mark schemes say credit other valid ideas: but unless markers ask for support, then things can be missed/squeezed.
However, while all this means not many grades change via the review, some do if unreasonable academic judgement (outside of the tolerance level and where markers have not asked for help) occur: and sometimes this does happen. There have also been some rarer instances where input of marks has been a keyboard error and also where marks have not been inputted.
So glad I teach at a uni where we don't have this going on!