So, I think the reason for this is how easy it is to cheat things like the 8mm Micros. I am not strong at all but can hang the micros for a similar amount of time by smushing my fingers into the back, so the skin rolls as I hang and there's kind of a pinch/friction effect. Way more than just fingers on an edge.
When you've got a slopey edge, so not incut, you can't cheat them, and it highlights the hole. Lattice use their rounded 18mm rung for this reason, you can't get away with cheating as easily and you have to focus on finger form properly.
I dunno, this is the conclusion I came to. I just do max hangs on 18mm edges now and focus on my fingers being properly placed.