A few years back we flew DFW/ORD/LHR and return LHR/ORD/DFW. Each direction had the same flight number from DFW to LHR and LHR to DFW. The flights changed aircraft type in ORD. MD80 to 777 and 777 to MD80. When the miles posted, the DFW/ORD/LHR posted as one flight leg but the LHR/ORD/DFW posted as two leg flight. Why? Never could get a reply from AAdvange on reason. The mileage was even different by a few miles also! Two leg got more than one leg.

The only way I've gotten to get the exact mileage is to upgrade on of the legs and not the both part on flights that have the same flight number within the USA. This way they have to calculate the mileage. Just depends if you want to spend time in the back. You could also book it as two seperate reservations. LGA/DFW and DFW/SEA, granted you'll get a different fare!