Help says...YMMV
For example, if the report is printed as of period 3, you would have the following calculations:
Beginning Balance Period 1 + Change In Period 1 = Ending Balance Period 1
Beginning Balance Period1 + Change In Period 1 + Change In Period 2 = Ending Balance Period 2
Beginning Balance Period 1 + Change In Period 1 + Change In Period 2 + Change In Period 3 = Ending Balance Period 3
(Ending Balance Period 1 + Ending Balance Period 2 + Ending Balance Period 3) / 3 = Average Inventory