Where do you see this screen in ME23N?
I guess you are double clicking the account assignment and SAP takes you into the asset master.
The F1 field help explains how the field is maintained and updated.
If it is not updated, then it usually has a reason like unit in asset master is different from unit in the document.
OSS note 1496720 - Quantity management and quantity specification in reporting explains further when a quantity is updated or not. See as well the related notes