Some of the time the calories burned reported for activities are bizarrely off. Today I went on a hike where the previous same route was calculated at about 400 and today it was 85. Same watch in both cases, using the HRM strap in both cases. I've also had the same thing happen a week ago where a short intense activity was dramatically underreported. Similar thing happened in Jan where a 4 hour hike came in at somewhere around 100cal.
Contacted support about it back in Jan but got given the run-around and responses are so slow that I eventually just hoped it was a one-off glitch. Seems like it's not.
Here is another post about someone having a similar issue: forums.garmin.com/.../false-calories-during-excercise