Apple Watch Is Not Counting Exercise Minutes Correctly

My Apple Watch is not counting my exercise minutes correctly. When I get on the treadmill or elliptical my exercise minute count will be zero for the day. I will start my workout at the exact time that I start the exercises tracker for that exercise, and I will workout for x number of minutes. I will presses the “end” button on the exercise tracker at the moment I end the exercise, and the summary will have the exact accurate count of minutes I exercised. Then, as soon as I press the “done” button, the watch will go back to the home screen where it will display an incorrect number of exercise minutes, usually about 3 minutes more than the tracker said. Why is this?