Active Energy in Apple Health app

Apple keep sending me notifications that my "Active Energy" has been lower for the last 18 weeks. I haven't done anything different in this time. I am still going to the gym 3-4 times a week and my iPhone is connected to my MyZone app which tracks calories while doing a workout.

When I check my calories, for example last Wednesday, it says 279 kcal. When I click into "Show all data" it shows that I burned 704 calories in a class.

I don't understand why it is so much lower? Can anyone explain this to me?