Difference in GR and invoice posting Prices with respct to Purchase Order

Dear All,

We have facing a problem while posing a MIRO for our client . The background is as follows for the Import PO (Document currency is GBP)

1) Rate of Exchange is 1 Euro = 0.84260 GBP....for August 2010 maintained in OB08

2) PO Rate for item A is 4.077 Euro/ Kg...PO qty is 1090 Kg...PO Date 05.08.2010

3) When GR was posted system post Amount at 2323.64 GBP (or 2757.70 Euro).....GR date 16.08.2010

4) GR Value should be 3744.4777 GBP ( or 4443.93 EURO)...which is appear when we post MIRO..

My question is from where system picks this rate i.e. (2.53 Euro or 2.1318 GBP per Kg)....No info record maintained and rate in material master at the time or GR is 1.7986 per Kg..Kindly help me to find out problem...

Regards

Sachin