Please explain me something:
in closed loop the PCM is constantly adjusting the fuel trims to maintain a Lambda value = 1 (stoichiometric).
Why should we have a lower fuel consumption with positive fuel trims? No matter what the trims are the PCM will still aim at Lambda = 1.

I belive you about the positive results, but why does it need less fuel? (-10% in average, that's A LOT!)