Why does car decelerate 5-7 mph when cruise control is on?

If the car is going up an incline, it will attempt to keep the speed constant with the setting of the cruise control. When that process is satisfied, the car may decelerate a bit to keep the same speed. If the car continually decelerates whe using the cruise control. stop using the cruise and have it checked. It id very dangerous to use the cruise control that is not operating properly. It could cause the car to accelerate unexpectedly.