The mystery deepens: I made two charges, one after the other, in a Brazilian supermarket on the same day 05/22/06 as the above mentioned ATM transactions. One was with Diners Club / MasterCard and the other CapitalOne “Go Cash” Visa. Both are credit cards. The exchange rates used were (calculated by dividing the amount of the charge in Brazilian reais by the dollar amount on the credit card statement):

Diners Club / MC R$ 2.20 to US$ 1.00
CapitalOne Visa R$ 2.29 to US$ 1.00

Both of these charges posted on the same day as the charges were made. Of course, Diners will add a 3% fee later on, effectively reducing their rate to R$ 2.14, for a difference of around 7%. CapitalOne will eventually rebate 1%, but Diners will give miles or points, so that may come out even.

Does anyone know why MasterCard and Visa could have different rates for charges made at the same time and the same place?