I recently watched a painter explain how imitating another painter is an impossible task even if you have exactly the same tools as them, simply because the way you paint depends not on what you use but on the way you think, the way you see the world, the way you move your hands, … Basically, the way your brain works. While the mechanisms of our eyes are essentially the same, maybe the answer to this difference is in knowing why we do what we do, why some brains decide to push for improvement in a given set of circumstances and not others. By which I mean, how much AF we do subconsciously, while our main focus is something that is not the blur. And alternatively, how much strain do we add to ourselves out of habit.
I have not been able to improve for quite a long time while wearing an undercorrection and knowing AF. Admittedly my phone use time was nowhere near 0, but I got back into EM specifically because of a time I lost my glasses and started going out without them (without any extreme change in habits) and by some point, I noticed improvement. I have no idea what prevented me from improving before that, and to be frank it annoys me because whatever it was, it was happening on a mostly subconscious level so I have no way to pinpoint it.
I have noticed though, that I tend to strain more when I wear glasses. During my first years wearing glasses, they used to give me terrible, terrible headaches and I had to push through and to “get used to them”. I associated being “mindless” of my body with wearing glasses no matter how undercorrected. As a result, unless I am intentionally being mindful of my surroundings, I tend to zone out and stop challenging my eyes if I have glasses on until I remember to do so. And so, maybe I would get 10 minutes of doing AF in a 1 hour walk? Meanwhile, I suspect that not wearing glasses gives my brain a cue to do AF on autopilot. A “no glasses so there is blur” cue so that the blur gets cleared even while I am somewhat lost in thought.
It is purely a theory, but I suspect that the “cue” for everybody is different. In all likelihood, it is a state suspiciously similar to when the first reduction/improvement was noticed. Some people forget to clear blur if there is too much of it, other people forget to do it if there isn’t enough of it. It is not so much about how much blur there is as how much blur your brain associated with the habit of continuous active focus.