Difference between Amount in local currency and Amount in Group Currency


My client's is having different company codes. One company code(ABC) is using GBP as a local currency anotther company code(XYZ) is using EUR as a local currency. They are using GBP as a group currency.

Co.code:ABC posted a goods receipt(gr/ir) in march 2009 for 90 euros. then system convert the euros into local currency as 75 and group currency as 75gbp, so we don't have any issue at this moment.

They raised a invoice verification(MIRO) in April 2009 for the above line item -90, then system convert the euros into local curreny as 80 gbp, and group currency as 82 gbp. Here we observed that, there is difference between local currency and group currency as 80 and 82. As per the rates maintained in OB08, group currency 82 is correct value.

Can anybody please let me know, why the system is showing different values for local currency and group currency, when both are using GBP?

Thanks in advance