I have noticed the same thing, I don't think it's anything to worry about. Maybe a safety feature so that u can't recrank it after already trying once and let go of the key. I'm not exactly sure why but I personally don't think its anything to worry about.
i think i remember reading elsewhere that since the g6's ignition is electronically controlled, a quick turn of the key is all thats needed to start the car. you don't need to hold the key to get the engine to turn. i had this problem too, but since i started switching the key in a quick motion it's stopped.