Margen
You pay £50 in London with your usual bank card. Your app shows a charge of €62.04. Using the ECB official rate of the day, you should have paid €57.95. The missing €4.09 lives in three separate places on your statement, or nowhere at all.
First piece: the currency conversion fee. Most debit cards charge between 2 % and 3 % of the amount; some credit cards go up to 4 %. On £50, that is between €1.15 and €2.30. It sometimes shows up as a separate line labelled «Currency conversion fee».
Second piece: the bank's own spread. Your bank never uses the official rate. It uses its own, between half a point and a full point worse. In this example, €0.50. It does not appear anywhere on the statement: it is already baked into the «original amount».
Third piece: the fee for withdrawing pounds from an ATM there. Usually a fixed fee (€3 to €5) plus a percentage. On £200, it comes to a clean €8.
How to see it tomorrow in five minutes. Take your last travel statement. Divide the euro charge by the original amount in local currency. Compare that number with the ECB rate for that day on fxtop.com. If the difference is more than 1 %, your card is charging you something you cannot see. Accounts like Revolut, Wise or N26 use the ECB rate with no markup up to a monthly cap (sometimes €1,000). Above the cap, they charge too.