A) What little level-of-difficulty difference there is from one exam to the next is offset by adjustments to the score curve. A tougher exam gets a more "forgiving" score curve, where it takes FEWER right answers to get a given score. A statistically easier exam compensates with a tighter curve, where you must get MORE right answers to attain the same score.
And no, you're not being assessed against the other people taking that exam on that date -- so it's not as if smarter people taking a given exam date hurts you or lower-scoring people taking another exam date gives you a hidden benefit. The score curves are established statistically, before the exam is even given, by the performance of individual questions in pervious years' "experimental" sections.
Finally, not even your %-ile ranking is measured against your own exam date's test population. If you notice, they always label that %-ile ranking against a running tally of all the people who took the test over the THREE YEARS preceding your own exam date.
There is ONE little exception to all this, though and that is:
the February exam is generally not published, so you don't get to
see your own performance breakdown as you would on any other exam
date. But February is such an off time anyway, this is usually not
a big deal.