I agree that grading standards, for the most part, are set by the professor, perhaps the department, and the university.
I have had classes with true curves (meaning that students may be graded down based on the curve, not exclusively up), positive curves (improved grades), percentages, and a variety of other grading schemes. Also, class size will often influence a grading scheme. Also, some schools that have multiple sections of a given class may have a specific grading scheme for that class (or use the same test across sections, etc.)