I am thinking you are drivng a red Kenworth t-600 or 660 with an ultrashift transmission in it. Even though this is considered an automatic, it actually is an automated stick shift trans.When you let your foot off of the brake pedal, and put it on the gas, and give it gas, there is a short period where the engine rpms have to be at a certain point where the clutch will engage.This is why it rolls slightly.It is the same setup as a muscle car with a 2,500 rpm stall converter in the automatic trans, where the car wont move on it's own until the rpm's reach 2,500 rpm's.This can be a real pain in the rear while backing into a dock as well.