Airspeed with the digital(!) sensor is very noisy, even in a wind still environment the sensor always shows around 4-8km/h, ours have always done this. It normally settles down and becomes fairly accurate once airborne.
Doing a once-off airspeed ratio calibration while loitering usually increases the accuracy (in the air).
I normally only do a preflight calibration if the airspeed is over 10km/h with a pitot tube cover on the pitot tube which ‘resets’ it back to 4-8km/h.
I’m not sure ARSPD_SKIP_CAL is a good idea, have never done or needed that in 500 hours of missions.