Because students on a higher starting level generally make faster progress (in line with the faster progress that they made at primary to get to the higher level at KS2) so comparisons are useless?
In a setted subject like maths, it becomes glaringly obvious how useless all these performance measures are. Want a good performance review? Nab yourself top set. Want to manage out a member of staff? Give them bottom set.
And how absolutely stupid is it to rate a teacher based on GCSE results against target or progress from KS2 when they have only taught that exam class for probably 2/5 of their secondary career?
How idiotic to rate a teacher on A-level results when at least 50% of the work is meant to be done outside of the classroom (ever told the kids that they need to do 1 hour of independent work for every hour they are taught?).
The system is cracked and it worries me that even apparently people like jaffa who have spent time, care and effort on their performance management system don’t recognise it.