He took the physics SAT II exam last month (May). It must have been a more difficult test than usual because the cutoff percentage needed for an 800 was lower than usual.
As an aside, I don't completely understand why the percentiles at each score range vary depending on the subject. For example, based on the table that I looked at, if one scores in the 92% in physics (for my son's test day it was 90%) that is worth an 800, yet in order to get an 800 in literature, one needs to be at a much higher percentage.
I think the reason that my son thought the SAT II was more difficult than the AP was that a few of the questions on the SAT II required rote memorization - i.e. either you knew the correct scientist's name or you didn't. Whereas the multiple choice questions on the AP exam required you to apply a concept to arrive at the answer. He prefers the AP type questions. That being said, the test prep material prepared him very well for the SAT II.