We had a pilot instructor manually add one of us as a student to his class so we could demo the student side of Gradebook for him. He gave us a grade, saw the student end of Gradebook, and then deleted us from the class. However, the grade he gave us on the assignment, even though we're no longer in the class, is still being averaged in to calculate the "Average for Scores Entered".
I think this is a real scenario at Michigan, since students can add or drop a course until about the 3rd week of classes. For example, a student starts a course, bombs the first assignment, then decides to drop the course before the drop/add deadline. They drop off the roster, and should drop off the gradebook, along with that data.