It appears that around 11:30am EST (June 4, 2015) Active Calories report went seriously broken. The number of calories attributed to walking skyrocketed. What used to be 200-300 KCal per day now is close or in excess of 1,000 KCal. What's more, this change isn't reflected in Total Calories report. Also, the days with no activities (walking alone) still appear to have a reasonably decent walking calorie assignment. So, my only theory is that Garmin started double counting something.
Perhaps, activities that started as "indoor walking" (kettlebells and indoor rowing for which I'm forced to use indoor walking option on my Vivoactive) but have been reclassified after being uploaded are counted twice? Or runs themselves? Or all activities and walking are double-counted as walking?
For example, last Friday (see attachment) I had 1,177KCal active calories according to the Total Calories report. But Active Calories for the same day shows 1,770KCal, with bulk of which -- 979 -- are coming from walking. Looking into day's details, we see that 6,026 steps in my run (540KCal) and 8,002 steps throughout the day (386KCal). So, the day details don't match Active Calories report either.