your credit card company always has different values for currency transactions (buying vs selling if you will).
An example:
1. If you buy something for $100US, the credit card company will charge you $135CAD on your statement (which is fine).
2. Get a refund the same day (hell seconds later) from the retailer.
3. The retailer refunds you $100US, the credit card company credits you $130 CAD on your statement (which sucks ass) due to their "conversion rates".
4. You spend 1 hour on the phone talking to 3 different people to get back your $5, which the credit card company ultimately decides to do as a "one time courtesy".
of course just making up the numbers above to illustrate my point. The difference will be significant, especially on a $1k USD purchase.