Why are GPAs calculated the way they are? At most schools an A is a 4.0, an A- is a 3.7, and so on. I feel like this system is not really representative of a person's true skill. For example, a person who got a 100% in a class will get the same GPA as a person who got a 94% in the class. However, a person who gets a 91% in a class will get a significantly higher GPA than someone who got an 89%. Why don't schools base GPA off the actual percentage that someone gets in a class instead of their letter grades?
The big reason: exact numerical scores are not comparable across classes and professors. 

Suppose I told you that two students took a class offered in different semesters. We look at their transcript and see that student A scored a 94% with Professor X and Student B scored a 99% with Professor Y. Student B beat out the first person by five percentage points, but do we actually know that Student B is a better student? Maybe Professor Y is a soft grader or Professor X had a really harsh curve that semester. We don't really know whether Student B is better. All we really know is that both students did pretty well.

Hence, grades tend to be assigned and interpreted with a large degree of subjectivity, which fits the ABCDF model better than a score-based model. The general interpretation is:

A - excellent
B - good
C - average
D - needs improvement
F - failing


If the first and second student both get A's, then this means that an expert in their fields (the professors) have said that both students did excellent. This isn't a perfect system, but it is about as good a comparison as you can get.