Where does My fitness pal get the exercise cals from?

Apologies for any confusion with your calorie adjustment from Garmin. Keep in mind that Garmin is not tracking individual exercise calories, it is tracking a complete day burn. Because of this, reaching your steps goal for the day may or may not guarantee a calories adjustment.

Your Garmin must estimate a higher number of calories burned compared to the set daily burn from MyFitnessPal, in order for an adjustment to be earned. If the number from your device is lower than the number from MyFitnessPal, then the adjustment will show as 0, or if you have enabled the negative adjustment option, then it will show as a negative amount earned and then deduct calories from your goal. When you do see a positive number adjustment, that number represents the amount of calories burned above your MFP goal and not the amount of calories burned for you workouts. This update appears in your diary as a Calorie Adjustment. The details of the adjustment's calculation can be viewed online by clicking the "i" on the adjustment line, or within the app by tapping on the adjustment.

When adding in manual exercises, or syncing single workouts from other app partners, those calories are automatically added to your set daily burn total on your MFP account and nutritional adjustments are immediately given. This can cause confusion when looking at your device adjustment because that number will lower or possibly zero out. This is normal because the number that your device compares its calories to (set daily burn from MFP) has now increased with the addition of new workout data and the distance between the two calories counts will be further apart.

Only when the Device number is greater than the MyFitnessPal number will you then earn the extra difference between the two as a positive adjustment.