Ah, I think Matt is just talking about the percentage scales used in individual courses... different profs will use different methods of collecting your grade during the course, and then they tell the class how they derive a letter grade (and thus, a GPA), from your marks in that course.
For example, I've had several profs who grade your exam, and you get a percent. And then, depending on the difficulty of the course, they assign those raw percent grades to letter grades. Because psych courses tend to be "easy", people need to score a 91+ average in order to get an A. On the other hand, in certain physical chemistry courses I have taken, getting a mere 75-80 gives you an A-, and an 81+ gets you an A. That's because in those particular courses, there were exams where over 60% of the class received 50% or below on the exam, whereas in a typical first-year psych course, the lowest grade might be a 75%.
That's how it has worked for most of my science courses. Then, in English, the profs tend to simply assign letter grades to tests and assignments, and the average of those grades becomes your final grade. The whole percentage thing isn't an official scale by any means, it's just a way for profs to account for variances in course difficulty, so that you don't get an entire class that all get A's, or C-'s.
Weird. I guess as screwy as UW is, it is pretty standardized. That is, grades seem to always be curved, and every prof seems to do it the same. I guess that is because percentages are what is ACTUALLY reported. You don't get a letter grade here. One of my majors is an Arts and the other one is Science, and in both the curving process seems to be exactly the same.