If lodged on top, the unterzug seem to very thin in vertical section, does it not? The ceiling is not used as a floor above? So no load other than the dead load. I was in hopes the planks set into a groove cut into the side of the unterzug.

It seem they were running low on trees and decided to use thinner logs for the side wall, they typically ran horizontally? No, the planks are substantial. Or how does the types of trees available determine how they built? Bigger trees yielded planks smaller trees gave log structure? Could soils, elevation, species and such things have an impact on building styles, to some degree.