Head Office / Branch. Head office not extended to the sales area from which billing is done

Hi friends,

In the Sales order  & Invoice the customer code 'X' is used as Sold to Party, Bill to Party, Payer.

In the customer master against this customer 'X'   Head Office  is maintained as 'Y'.

The accounting document is posted against the Head Office 'Y' even though the Head Office 'Y' is not extended to the Sales area in which the invoice has been created. 

Can someone please explain this  in the Head Office / Branch concept.

Is it not necessary for the Head Office to be extended to the Sales Area ?

Thanks in advance.

Regards,

Uday