25percentcpa,
My understanding of the actual grading (waves aside) is that each question has a statistical difficulty associated with it (some are hard, some are easy, most are in the middle somewhere) and that the way that they determine the difficulty is based off of how cadidates do on the question (hence why people with new simulations have to wait until wave 2, they don't have a large enough sample).
So the more people miss a question, the harder it is deemed to be. A missed hard question will hurt your score less than a missed easy question, but if you get it correct then it will help you more than if you got an easy question correct. Keep in mind this isn't a curve, a curve is a preset proportion of passing scores (say the top 45% scores of all exams taken in a window will pass no matter how they do). They don't do that.
As for why there is about the same percentage of passing scores in both waves, (once again statistics) there is a large enough sample in both waves taken from the same general population and roughly the same time period (think normal curve) that the passing rate should be about the same from wave to wave
AUD 81 - BEC 89 - FAR 82 - REG 86