Currency exchange rate & differs from PO rate & by & %

What causes this issue?

The currency exchange rate defined in the purchasing document is &V2&,
but the current exchange rate &V1& differs from this by &V3& %. (The
purchase order exchange rate can be specified in the header data of the
purchasing document).

System Response

If the PO currency exchange rate of the purchasing document is fixed,
you cannot change it.

Error message extract from SAP system. Copyright SAP SE.