Why is there such a wide calories burned varience doing the same exercise for the smae number of minutes on a day to day basis. vaience can be 200 calories or more.

I do an indoor elliptical for exercise. The calories burned according to the apple watch varies widely-sometimes 200 calories or more for the same exercise and the same time frame. Today’s calorie count was 279 and yesterday’s was 534. I don’t understand why there is such a big difference for doing the same thing.

is there something I can do to improve the accuracy of the Watch in regards to calories burned for an indoor exercise?