I don't know why one eye is more myopic than the other. It may be at least partly the result of having had an infection in it which resulted in spending about six months with atropine in it. This happened a long time ago, but maybe it was the start. Atropine prevents proper focus, which meant I had to get used to using only one eye and ignoring the other.
Nowadays, when I get good focus, I am aware that it's my right eye that is focusing, while the left one feels fuzzy, as though it was wrapped in a blanket and couldn't make contact with the world outside.
So I feel the left eye needs to be able to practise focusing - which it can do, as well as the right eye, as I demonstrated in the previous video. I can get 20/40 with each eye on its own. It's just when I use both eyes, the left one can't seem to do its share of the focusing work.