Exchange rate. There are now fees on crossborder purchases.
Different banks have very different fees for cross-border transactions. Before there were no foreign transaction fees but because of banks' appetite to generate more fees, some banks have started charging a higher than market exchange rate.
Some merchants convert your foreign bill into your home currency. Dynamic currency conversion (DCC)means the foreign merchant, not Visa, is converting the currency. As a consumer you see the USD value but you may be charged more for this foreign merchant service.
If you choose DCC for a transaction, you receive the merchant rate, not Visa's rate.