I wanted to find out what they actually wrote in fitfreak.net forum and this is the theory from member Bassguitarist1985:
"This is how the ECM is programmed. It will always default to a lean burn until called upon for more power, either first by throttle position then rpm. The stumble caused when the engine goes from lean to ideal fuel/air ratio, the few cycles transitioning from each mix is when this stumble happens."
I'd buy his theory.