I of course don't know, but suspect some sort of grading curve might have been involved here.
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Occasionally, student performance is lower than expected on a test. Instructors can choose to modify the student scores to equalize the scores.
A simple method for curving grades is to add the same amount of points to each student’s score.
A common method: Find the difference between the highest grade in the class and the highest possible score and add that many points.
Example: Curve a test score
If the highest percentage grade in the class was 88%, the difference is 12%. You can add 12 percentage points to each student’s test score.
If the test is worth 50 points and the highest score is 48 points, the difference is 2 points. You can add 2 points to each student’s test score.
You can curve grades in Blackboard Learn in two ways:
Method 1: In the Original Course View, download the test’s Grade Center column, apply the appropriate curving calculations in a spreadsheet program, and upload the resulting grades into a new Grade Center column.
Method 2: Manually override each student’s original test grade.
https://help.blackboard.com/Learn/Instructor/Grade/Grading_Tasks/Curve_Grades
See Bell Curve.
Most teachers have trouble with mathematics.