Aroc, you asked a very good question in my opinion. I believe that it has to do with each beekeepers climate. Here in N.C. I've read lots about this. I run 10 on 10 brood chambers, our winters are not harsh. Some hives (colony's) will move up to 2nd 10, some will stay down in bottom 10. Why i can't figure out. Not sure if it's genetics. But i beginning to thank that's the reason alot of beeks, use the 5 on 5 nucs. and 10 or 8 frame boxes with divider boards , don't know. I've not figured it out yet but will.