I think it's scaled by difficulty of the overall exam (i.e. a 65% on one exam does not equal the same board score as a 65% on another exam).
I don't think easy questions on your exam are scaled differently than hard questions though. So, in the case of 1 student getting 20 easy questions wrong and 10 hard questions wrong vs. a student getting 30 hard questions wrong on the same exact test....those individuals would still receive the same Board score.
In other words, hard questions aren't worth 1.5 points, and easy questions worth 1. Which is how I thought the OP was trying to discuss things.
I'm pretty sure the real exam is not weighted in terms of some questions being counted more than other questions, but the NBME practice exams may be weighted only because I have seen people get the same 3 digit score with different percent correct.