I found that the T&C a bit misleading. Amex charge 2.5%, Mastercard charge 1%.

But when you look at the effective translation, Amex worked out better.

Let me show you 2 transactions on the same day 13 November 2014. i was in Thailand.
Spent THB 175.82 in Central World, Bangkok with MASTERCARD - statement appear RM18.43 (which means RM1 can exchange for 9.5398 THB)

Spent THB 7,200 in Chiang Mai with AMEX - statement appear RM752.99 (which means RM1 can exchange 9.5618 THB)
The above transaction show that RM is worth more under Amex rate, had i used Mastercard to pay for the transaction in Chiang Mai, the statement amount would have been RM754.72.

RM1.73 or 0.23% more expensive than Amex.
In this case, i suspect upfront Mastercard charge 1%, indirectly charge another 1.73%, thats why end up 0.23% more expensive than Amex.
But  then again,  these transactions are not swiped back to back, they are swiped at least few hours apart from each other.