The chargeback amount can vary if the card's issuing country and the merchant country differ.
For example, an American shopper purchases online from a European merchant and checks out in EUR, their issuing bank debits the card in USD.
If the transaction is charged back, the issuing bank will exercise the chargeback for the same value in USD.
As the chargeback is always exercised later than the transaction date, the amount is converted according to the currency conversion rate of that day. The difference in conversion rate can result in a different chargeback amount.