Is this a bug?

Hi.

Yesterday after a run I noticed, that my Body Battery didnt drop a single point in 1 hour  - and later same day I walked for 2 hours and again not a single drop in Body battery.
Can someone explain if this is a bug or normal behavior of Body battery? Its my first watch, so i wouldnt know. The wrist band in tighten as it should. Did I do something wrong in the settings?

For better resolution of the picture: gyazo.com/f44c4bba251221b919a3379062b2bfdb