When I make a watchface, the phone display shows both the phone and watch battery levels as the same value. However, when the face is transferred to the watch, they show the correct values.
I'm guessing the phone has no way of seeing the watch battery. This isn't really a bug, adding watch battery to phone display is more of a feature request.
I would like a battery slider in the phone app, like the Time Machine feature, where we can manually adjust the battery % and see how it looks on the watch face while editing.