Robo - what brand of bees did you use?
My own feral stock
Do you have a theory as to why the hive would make them less productive?
Pure speculation, but I believe it is easier for them to maintain a vertical nest and work up and down combs than traversing across them. I know the winter cluster moves vertical much easier than horizontally. It just seems more practical for them to keep a vertical brood nest warm, since heat rises, than a horizontal one.