Account Assignment differentiation SO & Billing

Dear All,

I have created a sale order for material A with Account Assignment "Z1", once it's done, i go to next step of delivery and billing. the last step of billing is very strength cause Account Assignment of material A has been change its account assignment to "Z2" automatic by system which is different from Master data & SO.

Can anyone explain above this case? How to solve this issue to prevent next time?

Regards,

Rtn