account assignment group in Sales Order PROBLEM.

HI , ALL

I ' ve met some account assignment group update when material master was updated with account assignment group (MVKE-KTGRM) as blank (Sales Org 2 view), but some account assignment group no update in sale's order.

1. material was created with account assignment group (MVKE-KTGRM) as value "01" (Sales Org 2 view)

2. Subsequently Sales Orders were made against this material.

3. material's account assignment group(MVKE-KTGRM) was updated by value "02"

4. some account assignment group（VBAP-KTGRM） was updated in Sales Order , and the others account assignment group was not updated in sales order

5. the sales order 's create date are before the material' updated date

what might reasons for this...?