My guess is this is a "quasi-fuel-dump". AA isn't supposed to plate LY-only tickets on their own stock, so their system isn't designed to collect the YQ for those fares... buy since it's technically possible to book LY via AA's system, if you can get an agent to do it for you, you'll hit the jackpot.
If my theory is correct, this would work better with lower fare classes (where YQ is often higher).
Now, if only meshugener would share his record locator with me, I might know more...