Nevermind! I figured it out. You need to extend the grace period to BEFORE the tween reverses direction too.
After replaying and careful examination, I was tapping the key just before the ball hits the wall and reverses, causing a fail every time. The ones i hit were by cutting it closer and closer until i accidentally would edge over into the start of the reverse tween and suddenly score.
Once i noticed what was happening, and conciously decided to press down after the change of direction, the beat became significantly easier to hit. This also explains why i didn’t even realise there was a “good” until you mentioned it, because the only ones i would ever hit were perfects.
It’s still possible that my other two theories are amplifying of course :P but basically my hypothesis is that the people finding it difficult are pre-empting the change of direction.