Yea.. I take it that the garmin has the chest strap, so it is constantly taking your heart rate into calories burned.. and im sure the software that you set up, had you put in your weight and some other info.. its probably not 100 percent accurate (nothing really is), however, it will get your within 95 percent.
Unfortunately all of the numbers above are horribly innaccurate. The only real way to determine calories burned is with a power meter. After talking with some friends about the numbers their power meters were showing here is the closest website that worked for me:
Does the Garmin calorie counter use the HR or elevation data that the Edge is monitoring? I have a Garmin Edge 305 that has a terrible calorie counter. Simple formulas like 40 calories/mile are much more accurate.