In GBNG, click into the Grade Schema section to review the numeric ranges for the letter grades. Pay close attention to the editable column for MINIMUM grade per letter.
Now change the Grade Type from letter grades with +/- to Grade Points. Notice how the MINIMUM now for an A moves to 100. This means that students earning a 99% in the class will NOT receive an A, but an A-minus or a GPA of 3.67.
If it can be done system wide, I believe this numeric representation of the letter grade/GPA in the Minimum column needs to default to a number less than 100%, and once the grade type is switched to GPA, the numbers should reflect the MINIMUM one needs to earn that grade, and not the Maximum %.
If you want to test, enter a single grade item in your gradebook, and give each student a single point lower grade than the previous (i.e. Student 1: 100, Student 2: 99, Student 3: 98, etc). That is the easiest way to see how the grades are reflected in the chart.