Sudhan,
This question leaves a lot to be desired. You quote 'something' from 'somewhere', say you don't understand it, and then ask us to elaborate. You haven't even bothered to give us a context where this issue actually occurs, asking us instead to provide one.
I am not an expert in MM or CO, but this sounds like a scenario where the actual invoice value differs from the expected invoice value, under the condition where stock is valuated via 'moving average'. If there is less quantity of stock on hand than the amount of stock implied by the invoice to be posted, it makes no sense to post the entire variance to the stock account. This would lead to a misstatement of the true value of the inventory.
As I mentioned earlier, this is not my field of expertise. I suggest you speak to one of your local MM/CO experts, and ask them to tell you about account postings during the business cycle of purchasing, receiving, invoicing, and issuing under the valuation procedure 'moving average'. Or, just read the appropriate sections in help.sap.com, paying particular attention to invoice verification. FYI sap help docs are liberally sprinkled with examples.
Logistics Invoice Verification (MM-IV-LIV) - SAP Library
I further suggest that in the future, you provide actual examples for us to evaluate, and not just snippets of 'something' with no context.
Best Regards,
DB49