I remember years ago as a young teacher listening to a speaker from one of the exam boards describing how they went about putting an exam paper together.
Experienced teachers would submit potential questions which were considered by a group of experts, who would check that they were on the syllabus and make sure that they were of the correct standard for the exam and finally to make sure that they were unambiguous, grammatically and scientifically correct.
Then the board would select the best of these questions and put together an exam paper which would finally be inspected by another group to make sure that there were absolutely no errors.
This doesn't sound anything like the current set up. Having several exam boards obviously introduces competition, the result of which is to see who can offer the cheapest and easiest exams.