On S3 usually I'd burn 450-530 ACTIVE kcal during a 45-50 min HIIT exercise, with avg. heart rate ranging between 136-142. Then I bought S4 and the first time I used it I got similar results as with S3. However, then I decided to reset calibration data and recalibrate the new watch, because, well it's a new watch so I wanted to do the proper calibration. (I did two 20-min outdoor walks in Workout out in the open as suggested by Apple to calibrate the watch). After that, the next time I did similar HIIT exercise as before the watch tracked ONLY 167 ACTIVE kcal (!!) in 50 mins. How can that be possible? This is way too low. Heart rate was tracked alright, no gaps. Avg. heart rate 133. Today was the 2nd time after resetting watch calibration and the results after a similar HIIT training were slightly better: it showed 296 ACTIVE kcal burned. Avg. heart rate 140. However, this is still way less than before I reset the calibration when it logged 450-530 ACTIVE kcal during the training. Could it be that the watch is still in the process of "learning" and that after a couple more trainings it will display the kcal correctly again? Please advise. PS: for the record, my personal metrics in Health app are up-to-date.