Asset Value in ZOHO inventory valuation report

Asset Value in ZOHO inventory valuation report

Hi there,

I would like to know how does calculate the unit cost when we purchase an item and receive it from a bill.

Below is an example we have question with. 
As of 18th Sep, we have 7 qty remaining for item A with unit value $7.59.
We purchased  20 qty more, and received in stock from bill  9119T92726 part1, on 19th Sep. The unit purchase value recorded on the invoice is $12.61, however the unit cost in inventory valuation report shows $8.716.
Please could you advise how does the zoho calculate unit cost for an item in inventory? Thank you.