Re: vertical vs. horizontal movement
For colder regions I think it is thermal mass and conservation of energy. What little heat escapes the cluster will rise up and warm honey up above vs. below. If the cluster ate the honey in the downwards direction the honey would be colder and thus lower their body temperature. There would be no thermal mass above to temper the air temperature because the combs would be void. In the summer it may not be as important which way they go but moving honey for preparation for colder months would increase the work the hive has to do in fall so I think they have figured out what to do to reduce overall work to a minimum. This is all conjecture on my part.
Zone 5a, Practicing non-intervention beekeeping