The homework average in NetClassroom is displaying the average of all graded homeworks from the teacher's FAWeb gradebook.  It is possible that the teacher has graded additional homework assignments, but the show grade date has not been entered or still lies in the future.

For example:

The student sees three graded homework assignments that each have a grade of 100.  The displayed homework average is 92, which does not seem to match the displayed graded homework assignments.

In FAWeb, the teacher has graded two additional homeworks that each received a grade of 80.  The show grade date is next week so the student doesn't see them yet in NetClassroom.

So the homework average is 100 + 100 + 100 + 80 + 80 = 460
460 / 5 = 92

To correct the situation, the teacher in FAWeb could update the show grade date for the graded assignments so that they all display.