I think what he was saying is that the error distribution isn’t completely clustered in the corners so the height of the arms alone doesn’t fully explain the resulting errors. For instance, the first column stays very red nearly all the way down, then fades quickly to black. However, in the second column, the first point is black, and in the third column, the second point is black. If it was solely the height of the arms, then those patches of black do not make much sense and you’d expect things to more radiate out evenly towards the center. Also, why are the first and bottom rows nearly completely error free (except in the far corners), but similar results don’t show up in the columns (i.e., if the lower arms are keeping the machine in place in the top and bottom rows giving us all the dark dots, then why doesn’t it have any effect on keeping nearby errors out of their columns?). The graph definitely indicates something going on in those corners, but I don’t think the arm height quite explains everything wrong in the graph. It’s possible there is a combination of issues and not solely the arm height.