Per DK's hypothesis, the added bias would be a decreasing function of underlying skill, not a constant as it is here. I think this is the only way to define it

Per DK's hypothesis, the added bias would be a decreasing function of underlying skill, not a constant as it is here. I think this is the only way to define it sensibly. It's easier to see what it's doing if you look at the scatterplot, although it's kind of annoying because the axes are flipped and it keeps the view centred on the data, so the points stay still and the axes move around them. Basically it's just translating all the points uniformly along the estimated score axis.