With cruise control armed
elements like air resistance, road gradient tend to decrease the car speed continuesly. On lower speeds engine power compensates but at higher speeds when engine is already delivering the max the effect is more noticed.
The close loop system detects the speed loss and tends to regain it by changing the transmission configs in CVT and down shifting in planetary gear auto trannys.
The bigger cars which have more drag coefficient tend to go in continous loop of downshifting and regaining.
