I think it's related to how irregular the shapes of the letters are. The circles are all slightly bent out of shape in different ways, and the line strokes all

I think it's related to how irregular the shapes of the letters are. The circles are all slightly bent out of shape in different ways, and the line strokes all have slightly varying slants. Most attempts at making a font try to make all that stuff as regular as possible, even the one in the article we're discussing does this.