astanley makes a very interesting point that I had to think about for a bit - it all has to do with the time value of money - in other words the discount AA places on cash in the future when comparing it to cash right now.
For a $100ish each way LGA-ORD deep discount fare bought two months in advance to be worth the same (or even .66x as the EQM/EQP ratio would suggest) as a $400ish each way H fare between those two points bought on the day of departure, AAs discount rate for revenue would have to be extraordinarily high. This is commonly associated with companies on the brink of failing to meet fixed payments, who are thus willing to pay any rate of interest for cash right now. It is one reason why the interbank lending rate reached the levels it did at the height of the liquidity squeeze.
Obviously there are other factors at play here as well as time value of money, but perhaps the reason that BA gives more benefit to late-booking high-fare flyers than AA does is that it is in better financial shape?
I apologize if those figures are wrong, I have an AAirpass so I may be off by a bit.