I am not a medieval stonemason but these were lifelong craftsmen and I doubt they did it without a lot of thought. They did it with one specific thought it their mind I suspect, it was the way that they were most familiar with doing it so it was the fastest / easier / cheapest way to do it! The same reason any modern craftsman does quite a lot of things.
I'd like to see that 70% distribution mapped out against corner adjacency. As in, if I have a square room [ ] and I put a stairway on the right wall in the bottom corner there, there is a good chance I'm going to go counter clockwise, but if I put it on the right top, I'll go clockwise. I feel like the general flow of a room needs to be mapped out on this as well to get the full context.
> these were lifelong craftsmen and I doubt they did it without a lot of thought
There are so many things that could bias this result. Maybe it's easier to craft in one direction. Perhaps, north of the equator, castles tended to be constructed in certain orientations, with the windowless stairwells tending towards one side versus another. Altogether, there is no reason to prefer the myth over any of those hypotheses--they each have no evidence.
