This is because I am just a practical dude and so take advantage of existing Lang stuff (going for the cheap/easy/quick ways).
When retired and kids grow - will see.
But here is another idea to chew on:
1)we established how the swarms prefer 40-60 liter cavities the best;
2)we also established how typical tree cavities are about 30cm/12" in diameter;
3)do a calculation and you will see that an approximate cylinder with 30cm in diameter and 40-60 liters in volume will be about 50-60cm in height (60cm ~ 24").
And so, it would be normal and natural and desirable(???) to build vertically uninterrupted combs starting at the static upper base and downwards somewhere in 50-60cm range .
It maybe after doing this for ~30-40M years, the bee memory has been wired in and is very strong (for the forest populations; the savanna/mountain populations could evolve differently).
My setup is probably approaching the "desirable" span of a forest bee, and that what is going on.