Inventory Part- Last periodic Weighted Average date

Hello, 

According to the help text of the Last periodic WA date field in inventory part > Costs > Periodic WA Info Purch it says this field is getting updated after running the job Calculate Periodic Weighted Average. 

Last Periodic WA Date Purch:
The date on which the value in Cum Price Difference field was used to calculate and set a new periodic weighted average cost for the part. This value is read-only, and is updated if you run the background job started in the Calculate Periodic Weighted Average dialog box.

I created a purchase order for this part and received it. Manually supplier invoice has also been created and posted. 

After running the job Calculate Periodic Weighted average, Cum price difference field has been updated but not the Last periodic WA date Purch as below. 

I need to understand in which instances the field Last periodic WA date Purch is updated. 

Any clue is highly appreciated.