Third Party Workout Apps vs Apple Workout App

I recently switched from using the Workouts++ app when I record a run/walk/hike to using the Apple Workout app. I have noticed that a similar workout in the each app results in a significantly higher number of active calories being burned in the Workouts++ app than the Apple Workout app (around 15%-20%). I have noticed the same think using other third party apps, like the Nike running app. I have also found that the Fitbod app credits me with even more active calories burned than the Apple Workout app when I compare those two. It's not a huge deal if I just stick with Apple's app, but is there any particular reason this would happen? I figured all of the apps were basically using the same heart rate measurements to calculate calories.