Airspeed in Guided Mode vs Auto Mode

I’m using the TRIM_ARSPD_CM parameter to set my target airspeed in Auto and in Guided mode. In auto mode, the airspeed fluctuates ~0.2 m/s while in Guided mode the airspeed fluctuates ~1 m/s. Is there a more accurate way in guided mode to hold a target airspeed?

Does the airspeed fluctuate if you are in guided doing a loiter point? There should be no difference at all for airspeed accuracy here, if I had to guess you are comparing your performance of straight and level flight with a flight mode that is always in some degree of bank).