Calories burned during exercise

[LMK47]

I’m so confused...I workout first thing in the morning and just before starting my workout I’ll put on my Fitbit. I set the Fitbit to “workout” mode so I can track the calories burned while exercising. What I don’t understand is the total calories burned in the workout mode and the calories burned on the Fitbit itself, are completely different. For example, I exercised today for a half hour and had my Fitbit set to workout mode. At the end of the workout, it said my calories burned during the workout was 180. But then when I look through the main screen and go to calories, it says I’ve burned 620. So which is it!? 180 calories or 620?? The Fitbit was put on right when I started the workout so I couldn’t have burned any calories prior. Anyone have any insight on this? I’d love to know how many calories I’m actually burning during my workouts. 
Thanks in advance for any help!! 

[Cristhiane]

I’m not sure if I’m right about this but I think what’s happening is that it’s counting your nasal metabolic calories - the calories you burn just by being alive

[LMK47]

I think you may be right, Cristhiane. I noticed recently that before I even put the Fitbit on it already has calories burned logged on the Fitbit. It’s interesting though because it’s always a completely different number. Sometimes it’ll be a low number like 90 calories and other times It’ll be way higher like 400 calories. So I don’t fully understand how the number is calculated but it does make sense that it would be nasal metabolic calories. Thank you for your help!