I have changed the displayed currency in Settings and think the resulting exchange rate is different from the market rate
The billing system by default uses Euro as the main currency. All calling rates are counted in Euro; balances are kept in Euro as well; you only change the currency in which the balance and rates are displayed hence the name “[displayed] currency”. You do not run any exchange risks.
To see the absolutely accurate billing information, please set Currency in Settings to Euro otherwise do not worry about the exchange rate. The displayed currency other than Euro is used only for indicative purposes.