AFAIK, all American schools gives transcripts with letter grades, not percentages like some Canadian schools. The letter grades vary greatly between courses, with some being graded with strict 90/80/70 cutoffs for A's B's and C's and some being curved. Depends on the course/professor.
AMCAS then looks at your transcript and converts the grades to a numeric GPA, sometimes differently than the transcript does. A = 4, B = 3, C = 2, etc. I think that as far as AMCAS is concerned +/- grades are A- = 3.7, B+ = 3.3, etc, regardless of how your school calculates it, but I'm not sure.
So in American schools (especially those without +/- grades) you can get 89% of the points in a course which isn't curved and get a 3.0 (3.3 if your school had half grades). According to the
OMSAS conversions, that would have been a 3.9 if you had a transcript from a Canadian school such as U of T which listed 89% on it. Likewise, here you could get 79% and get a 2.0, while at U of T that would 3.30.
I'm really glad I took the time to look into this. Now I can cite this post every time some tuque-wearing hosehead comes in here and whinges about American medical school matriculants with low GPA's.
Edit: In retrospect, I'm not sure if I understood or addressed your question. I think it's a good answer, but possibly to a different question. Are you asking about GPA in medical school? Many schools are pass/fail, and those that aren't pass/fail have averages far lower than 3.5 -- that would be like everyone getting high pass/honors.