Originally Posted by MrDaniel

This is nothing the processor can control, in your case you have a Canadian issued card and buys things i USD, then in the authorisation a booking is made on the account with a higher exchange rate to cover for exchange flucturations between the auth and the postings on the account. This is something that is done after the procossor has forwarded the transaction to the issuer.

When the final transaction is settled the exact exchange rate is determined and is often less than the auth exchange rate, in rare cases of high flucturation in exchange rates it could be higher and the issuer stands the risk that there is not enough money on the card to cover the transaction.