Originally Posted by
stvr
PS: I would love someone to run a similar test with Discover Card!
Why? Discover is basically accepted nowhere, so it doesn't matter. (It's accepted by merchants which accept Diners, which is basically just major hotels.)
Your test of Amex is probably insufficiently substantial. Amex is extremely variable in my experience, possibly due to their stated policy of using "The highest interbank rate we identify from customary banking sources, on the conversion date or the prior business day." That leaves a lot of wiggle room if the currency happens to be moving about on the day prior to and day of your transaction being posted.