Calories burned during runs are suddenly inaccurate

The calories burned during some activities have gotten increasingly inaccurate. I track my runs on another app which, in the past, normally lines up with my Garmin. The Garmin calories have been steadily decreasing and today it showed half the number of calories burned as my other app. It shoes I burned 400 calories on a 9.5 mile run. The calories for cardio workouts have also been inconsistent. What can I do to fix it?