My best guess without beeing able to examine your application is that the issue lies in quantity_delivered. Does the field reside inside the same table as unit_quantity? Is it an one-to-one relation between the row unit_quantity and the row quantity_delivered?
Do you get expected output when using
Sum (unit_quantity * quantity_delivered)
And when using
Sum (TOTAL<CalendarMonthName_C>(unit_quantity * quantity_delivered))