Cheaper flights have the following restrictions in the fare rules:

IF THE FARE COMPONENT INCLUDES TRAVEL BETWEEN MIA AND LAX
THEN THAT TRAVEL MUST BE ON
ONE OR MORE OF THE FOLLOWING
ANY AA FLIGHT OPERATED BY AA
BUT NOT ON BOEING 777 AIRCRAFT.
AND
IF THE FARE COMPONENT INCLUDES TRAVEL BETWEEN MIA AND LAX
THEN THAT TRAVEL MUST BE ON
ONE OR MORE OF THE FOLLOWING
ANY AA FLIGHT OPERATED BY AA
BUT NOT ON BOEING 777-300ER AIRCRAFT.
AND