I have been noticing this a lot the last few days.
Last night I did a Body Attack class. Garmin tells me I burnt 414 kcals. Now the Previous Monday Garmin tells me I burnt 562 kcals.
The two classes did the same routine and were the same length in time and I felt I worked harder this week than last week since last week I had a massive blister on my foot from a Cross Country race the Saturday before.
Saturday passed I ran 27 miles in 03:52:30 and Garmin said I burnt 2,027 kcals. Which I felt was rather low for that length of run.
What I have been noticing is that the Active Calories on the app changes every time I go on.
Like on Friday I walked over 15k steps (to, around and from work) then ran 10 miles. Apparently I only had 957 active kcals for the whole day!
Basic stats. I'm Male, 5'9", 80 kgs, Resting HR of around 40 bpm, Max HR of 188 bpm (calculated using an online tool recommended from this forum) and I only turn the HR monitor on for activities.