Moodle has a default letter grade scale which may or may not reflect the scale of your course letter grades.
You can very easily change the scale to reflect your course.
First, under the Settings Block, click on Grades.
Next, under the drop down list at the top of the grader report, click on Letters-View.
You can view the letter grade scale to see if it matches your course letter grade scale. If not, click on Edit Grade Letters at the top.
Next, click on the check box to override site defaults. You can now change the letter grade scale to reflect your course letter grade scale.
When you are finished, make sure to hit Save Changes at the bottom.