Debit Memo Request wrt Invoice has different rates

Hi Friends

I am creating a Debit memo request with reference to Invoice.

The rate in Invoice(01.03.2012) is 100; but while creating Debit memo Request with VA01 on (01.06.2012) with refrence to Invoice(01.03.2012); the rates updated in the Debit Memo Request is 110 .

It is statistical but used in prinouts.Please help.

Regards

Praveen