Its a standard behavior...
Because of your fixed indicator is ticked in PO... So the different exchange rate amount debited from price different amount and goes to exchange rate diff account...
Check the thread for more details Exchange rate diff at the time of GR | SCN
And its goes to exchange rate diff account because of the setting on path
OLMR-Incoming Invoice-Configure How Exchange Rate Differences Are Treated
Check table T169P, depend on the value for field KDHW.