Lux Flyer -- yes, I understand the seat pitch is what AA is referring to.

If there are other material differences (besides seat pitch) between a Main Cabin Extra seat and the regular Main Cabin seat, AA should disclose them. In my case, yeah, I got 4" extra seat pitch, but lost about 1/3 of the under-seat storage, which resulted in losing the legroom advantage.

On the other hand, as per Kawliga's post, if I had a differently configured plane outbound vs return, that would explain it and my complaint is baseless. The flights I had were Boston<=>LA so I suppose it's possible.

Thanks for your replies!