Tax classification in customer master and billing document

Hi

I have an issue while validating tax on an invoice.In ship to party customer master tax classification is 0 while in invoice document it is displaying 1.Payer is not in the exemption list,it is a foreign customer and is not taxable.Tax code is O1.In a similar invoice tax classification in customer master is 0 and in the invoice it is 7 which is correct for foreign customer with tax code AX.

Can somebody please suggest why in the earlier case it is giving wrong tax classification '1' in the invoice which is '0' in the ship to master.

Thanks & regards