After passing numerous prep exams, including Rita's, I failed my first real PMP exam.  I called PMI, and while they would not use the term, apparently some questions wieighted, are given more credit than others.  It used to be a straight 61%, but no more.  Here is a quote from an email they sent me:

"Scaled Scoring is not a percentage-based scoring system, that is, the assessment passing point is assigned to a point of difficulty instead of a number or percent of correct responses."