Fitness app calorie count reduced

I use a third party app for HIIT workout. The app records my activity (duration, steps and calories burnt) and sync the data with Health app and Fitness app. As soon as the workout completes, everything appears good. The same calorie count is displayed in Fitness and the 3rd party app, and I could see that I closed the fitness ring. However, after some time, the calorie count was reduced in Fitness app. Why is that?

I don’t have an Apple Watch, but I do have a band that tracks my steps. I understand there may be some double counting if I use multiple apps to track my steps and workouts, but shouldn’t Health & Fitness take the maximum of all source apps, if not the sum?