I've just finished a long cycling activity (650+ km, 28+ hours) and was surprised to discover that my active calories value is 771 (that's less than I usually spend during an 1 hour activity). How is it possible? Activity link: connect.garmin.com/.../19436249032
The activity shows you've spent 75% of the time below the HR Zone 1 (warm-up). When your HR does not even reach the Zone 1, you burn very little Calories (at least theoretically), so there may lay the problem. Do your usual rides also show such low percentage of HR in the Zones 1-5?
As I can see now, the HR data (time in zones, max and avg HR) is also incorrect. Due to your screenshot I haven't reached Zones 4-5 at all, and my HR haven't exceed 149. But the pulse graph clearly shows intervals of HR in Zone 4 (with peaks in Zone 5) lasting for more than half an hour. 
Moreover, the activity has been synced with strava and there I can see completely different numbers: 
