Found periodic transaction amount between periodic reports are not same

- Periodic_Item_Cost_Change_Repo_210415.rtf  Pls see column amount of line   
       " Cost Owned Transaction(s)" that show 77,982,839.86 (this value used to calculate unit cost).

- Periodic_Material_and_Receivin_280415.txt Pls filter Source Type  "Purchase order" that will found 
        amount of this report equal 79,791,049.69. Program should be use this values to calculate unit cost or   
        not ?

But I don't know why system is not use amount 79,791,049.69 to calculate, because this amount is a "Costs added in current period" to inventory.
Why system used "77,982,839.86"? and this amount can't trace to transactions.