why is there a differences in Document Currency and Group Currency?

Hi All,

Need you inputs on the beow mwntioend issue.

We have raised a purchase order, Goods receipt has been done and Incoive is also raised. The  Document currency, Local Currency, Group currency (Curr type 30) and profit centre currenct (curr type 32) are all USD.

The vendor invoice is posted and we saw that the Local curr and Document curreny are same . but the amount posted to Groupd curreny and PC currency are different form that of Local Cur and Doc Curr.

When all the curreny types are USD, then how can system posted different values?

Regards,
MG