Limit PO Confirmation allowing to change account assignment

Hi,

SRM 5.0.

I have created Limit Purchase order with "XYZ" and going to make confirmation, while doing confirmation system is allowing to make changes in to account assignment ( cost centre)

user dont want to this to be change, as the purchase order is created for particuler cost centre, why system is allowing to change cost centre and allowing to add more account assignment in confirmation

waiting for reply,

Regards,

Jairaj