Body battery strange

I have some troubles understanding how body battery works when you have times not wearing a garmin device with body battery capabilities. Yesterday I didn't wear the vivomove for 10 hours (8:00 - 18:00, I went with my forerunner 645) - my body battery graph ends at 8:00 by about 95%. At 18:00 the graph starts with 26%. Obviously my body battery went down about 70% without wearing the watch (taking some info from the forerunner? or not?) ... so far so good. This day I went for a run not wearing my vivomove, also the FR ... my body battery stayed at the same level. Hmm. 

I think body battery could be a really good measure. But it seems that it's "out of tune" when you mix devices (at least with non body battery devices). That - of course - makes it less attractive.