I don't think it's 100% correct to say stuff like "someone who got x percentile last year would drop by y percentiles this year" or "I would've gotten a higher percentile if I sat the exam last year", because if you really DID sit the exam last, your overall score would probably be lower too, maintaining your percentile. I guess it's no longer a matter of improving your score each time you set, but improving your score FASTER than everyone else - everyone's improving, and you gotta keep up with the competition to maintain your standing.