No. Like many higher and professional exams, they are standard set on the basis of expert judgements. It is inevitable that exams vary in difficulty at the moment*, and this must be taken into account in setting pass marks and grade boundaries. These are essentially social constructs, but nonetheless seem to work - GCSEs for instance are quite good predictors of later academic performance.
https://www.gov.uk/government/publicati ... ed-to-know
*my tribe has always been interested in using Item Response Theory and other approaches to identify the absolute difficulty of items, but of course that only works when an item has been used before.
https://en.wikipedia.org/wiki/Item_response_theory
Not the best wiki article, but then I'm supposed to be at work...