Originally Posted by
Upgraded!
I assure you, you will never, and I mean never, see a situation on AA where a flight shows J7 D0 other than a mistake. While your theory may apply to award tickets or upgrade inventories, this is clearly a mistake. Since the price of D is often close to J and doesn't double as an upgrade inventory (like A does) AA will typically show as many D as J seats until you're down below J3.
It's not uncommon to see a situation where J is 3 or more greater than D, even when there is "plenty" of J. I see some J7 D4 and similar on JFK-LAX. But your point is still valid; this has to be a "mistake".