Issue is when it’s a thru fare. What seems to be happening — and it isn’t exact — is AA takes the mileage portion first and determines how many miles u get from that and then takes the balance of the the miles u would have gotten if it was all AA thru fare and subtracts the partner portion of the thru ticket. Net: it seems like on a mixed itinerary with a thru fare, you end up close to what you would have gotten if it was all AA marketed. Of course YMMV. This is not an exact science.
Originally Posted by
USFlyerUS
Correct. If you could see the underlying details of your ticket, there should be a fare calculation line that breaks down the value for each leg. It's not simply based on distance as the fare itself matters.