Most colleges have adopted a 4-point scale, with a 4.0 for an A, 3.0 for a B, and 2.0 for a C. This leads to a point of contention when some professors are willing to award A+ grades for exceptional work in a course. Most colleges do not give additional points above 4.0 for an A+ versus an A. This means that a student with a grade as low as a 92 can receive the same 4.0 that a student with a 99, 100 or above was awarded.
Conversely to the issue with A+ grades is the A- in the 4.0 scale. While an A+ garners no additional points over an A, an A- can be as much as .25 points less than an A, or a 3.75. This leads to an issue where there is no grade increase for as much as 8 points above a 92, but there is a significant grade decrease for a single point below 92. Some have argued that this encourages exceptional students to do work that is just good enough for a 92 and never press for that additional 8 percent.
With most large universities having multiple sections for popular classes, there is frequently a continuum of difficulty across sections. That is to say, there are usually known "difficult" professors, and known "easy graders." Consequently, the easy graders fill up quickly and the difficult professors are left for students who are late to register, or sophomores and juniors with later registration dates. This raises a question as to how comparable a GPA is between two students, with difficulty levels for each individual class varying, potentially widely.
In each course, you must ultimately be given a letter grade, which will correspond to a number grade in the 4.0 scale. This means each assignment has to be weighed by the professor as a part of your whole grade. Differences in these weights can lead to situations where individuals that excel in one area, perhaps paper writing, but struggle in another, say midterms and finals, have widely different outcomes.