Calculated calories from cycling.

Im using a fenix 5 and a edge 810 during my rides.

Im also using the vector 3S.

As an ex. my last ride i had an avrage hr at 150 bmp. And The ride took 70 min, wich should have been around 800-900 kcal. But mycket garmins says 620. Wich is to low. 

A presume that garmin is looking at my avrage wattage (150W) and makes a calculation from that data and not from my pulse. 

I look at similar rides: 70 min/150bpm/150w avg, and are hitting around 800-900 kcal. 

Anyone who can explain this? Garmin support doesnt understand this issue. 

Best regards Jonatan.