What the fee usually includes
A foreign transaction can include a card network fee, a bank or card issuer fee, and a currency conversion margin. Some cards show one combined percentage, while others hide part of the cost inside the exchange rate.
Why the statement can look different
The exchange rate used for your statement may be set on the processing date, not the purchase date. Refunds can also use a different rate, so a returned item may not equal the original amount.
How to estimate it
Start with the purchase amount, add the card fee percentage, then add an exchange-rate margin if your provider does not use the mid-market rate. The result is still an estimate, but it is far closer than a plain currency conversion.