Net value in milestone billing not matching with contract value

Hi,

My business is facing one issue where the Net value of contract is some 3800 KWD. and the milestone billing net value is 3500 Kwd.

I don't understand why this issue of price difference has come now. I checked other documents in SE16 (VBAK table) with same document type and billing type, they are picking the same value. No data inconsistency found.

Please guide what could be the reason?

I tried to cancel the billing document and redo the billing, then in update button select option D.

But that option it self is not available!