EUR/USD rate

Could you explain why the current rate for EUR/USD proposed by the application is not the best? There is a difference (not favorable for custemers in both rates) of 0,4% for EUR/USD rate and 0,6% for the USD/EUR rate. (Compared to XE rates).
Revolut pretend to apply the best rates for exchange but it is obviously not the case presently.
Any explanation for that ?

Weekend rates when markets are closed are poorer.