I'm not sure if I've done two foreign transactions on my SPG Amex on the one day before, but I did recently.
Same currency. Same date online.
First processed at 0.9975 x xe.com rate.
Second at 1.0038 x xe.com rate. (i.e. 1.006311 x the first transaction's rate!)
When Amex can vary this much during a single day (or, at least, what Amex claims is a single day), you need a good number of transactions to make predictions from.