I'd imagine they were built in a direction without a ton of thought, and it's not really surprising to me that if you basically flip a coin - 70% of them landed one way.
If the staircase happens to be on one side of the building, it would be better to orient it one way - based on how the windows would work - for example.
If the staircase is against another busy entryway - it might be better to have the stairs go one way or the other, etc
If there are several thousand such towers in Europe, then chance of 70% of them all being clockwise due to use of coin flips seems unimaginably small. More likely the first one happened to built that way and most others copied that until someone wondered if you couldn't go the other way instead. Or maybe it is connected to handedness in some other way (easier to draw the design? To lay the stone work? Traditions around who should pass who and how if somebody going up encounters somebody coming down? Who knows...)
I think it would be more likely that the staircase fit better going one direction than the other. Eg taking the most used path to the staircase and going straight onto the stairs vs having to do a 90 degree turn before you can step on the stairs.
It seems unlikely that random chance would lead to a 70/30 ratio for "best fit" either, so that just seems to be moving the phenomenon that needs explanation to layouts around stairwells, which certainly weren't standardised in the medieval era.
The ratio of staircases in Norman castles was more like 20:1 which is a ratio even more in need of a non-chance explanation; the greater numbers of anticlockwise staircases came later, when coincidentally or otherwise individual towers were less important to the overall defensive scheme
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.
But just that in some direction maters for some reason (layout of the castle forced a direction and thus this works out to random chance?), and the rest was just what the mason felt like building (default to right handed?). You can come up with your own story about why direction would matter, and why for the rest there would/would not be a bias in direction. Then play with how random each one is to get the 70/30 percentage split.
If the staircase happens to be on one side of the building, it would be better to orient it one way - based on how the windows would work - for example.
If the staircase is against another busy entryway - it might be better to have the stairs go one way or the other, etc