WIth Tim I was investigating the deletion of attempts from the quiz and their reflection in the Gradebook.
Screenshot 1 shows the Grade book as seen by a member of Course Staff immediately after the deletion. The deleted item was Susan Pearson's attempt at iCMA04. You will observe that the average for the column is correct.

Now look at screenshot 2 which shows what Susan Pearson's tutor now sees. The grade has correctly disappeared but the averages have not been recalculated.