In my experience the z6 uses a lot of power just to stay on. If shooting in a situation like a live show, with lots of images in 60-90 min, the performance is very good, easy north of 1500 shots with battery to spare. Walking around is a different issue. I usually turn int off and on again just before the shot to extend the life as much as possible. In this situation, and specially if I´m using manual focus, taking more time and using magnification in the evf, reviews, etc...it can drop to 300 before it depletes a fresh battery. I guess the cipa takes time on in consideration in their test?
Cold weather should not help either.