requisition for service and material procurement for mexican plants

Hi SAP Gurus,

PLease tell me why is this happening? When the mexican plant tries to create a service requsition( when the account assignment is K, Item category is D), the services tab and valuation tab get automatically populated and geryed out with MXN currency. But this does not happen for material proucrement(account assignment K, Item category -blank), the system gives us the option of changing the MXN currency on the valuation tab of the requisition. Why is it like this?

Thanks

Anusha