MOT's are carried out by thousands of different testers who will all have their own level of competence and interpretation of the rules so you will not get a consistent approach. This is particularly the case with advisory items where one persons judgement at what point to flag this up may be different to the next ones. You also have to look at the garages them selves, some always find something whereas others find far less. This may be down to judgement or the desire for more work.
The profit on MOT's is not great, but the work it generates is the real draw.