The sign of a credit to an account is negative for the purposes of affecting the account to which it is applied; similarly, the sign of a credit refund is positive.
In the report Cashier Transactions by Payment Method - Negative Amounts USD, the charges in local currency of the credits are presented in the report with a positive sign for the purpose of adding to the accumulated amount of the payment method although their real sign is negative, in the same way if there is a credit refund it would be presented with a negative balance to reduce the accumulated amount of the payment method.
As the amounts in USD are only shown for information purposes and do not affect the totals of the report, they are shown with their original sign, which is negative for credit memo debits and positive for credit memo refunds.