Actually this would be a NDC issue, specific to AA, which I almost never book, if you see a lower fare on AA.com but not on other channels, Amex included. I usually only book UA, AC, DL, and can often match the lowest fare on Amex, sometimes through the use of booking gymnastics by creating a multi segment itinerary, but quite often, as in the case of the itinerary that prompted me to start this thread, I’ll never match it with Amex, not even close. Amex will display a higher fare, and sometimes a random dollar amount that I can’t find anywhere else.