There is a bug where a letter grade is not calculated correctly due to floating point division. (floating point division by reciprocal multiplication).

Reproduction Steps:

1. Create a course with a total of 331 points.
2. Assign a student points totaling 297.9.
3. Make sure the grade scales is set to A- for a 90% or above.
4. The 'real' grade value shows 90%, however the letter grade is B+.
5. It appears PHP is calculating this similar to Python.
   >>> 297.9/331
   0.8999999999999999
6. This should be .90. 
   1. 297.9 / 331 equals exactly 90%.

There are a number of tracker issues open related to rounding decisions. However, this case seems to be a legitimate bug related to floating point division rather than a choice of rounding vs. truncating calculated values.

Docs which need updating if this patch is accepted: https://docs.moodle.org/402/en/Gradebook_calculation_changes