The writing position you describe, if I visualize it correctly from your description, is one of a couple of positions taught to lefties "back when ..." ... (by the relatively few enlightened enough not to try to force them to change).
Writing with a "wet ink" pen, or even with early ballpoints, it was about the only way a left handed person could avoid dragging the hand through what was written before it had time to dry enough to resist smudging and becoming illegible. The positions that some call "more normal," have the same smudging problem even with a pencil, if the lead isn't fairly "hard." The "curled" position is also well suited to being able to see what you've just written, without having to move the "pen" (or whatever) away from where you'll continue writing.
The reason they were taught is because they were close to what was "natural" for the kids trying to learn worked out as "the best way to get it done."
Your books were probably written by imature illiterate children (i.e. any people under 50, who grew up with "progressive" education and hence didn't learn much?) who don't know any traditional lore very well.
Maybe they should get out of education and take up a religion calling where "explanations that aren't" are more universally loved and appreciated.