It looks like they limit the radius of wilderness to the US border. The circle for Boundary Waters would be bigger otherwise, since the Quetico (on Canada's side) is wilderness as well.
From the article: "Wilderness areas in Canada are not taken into account. There is no Canadian national wilderness designation program like there in the USA, and determining the boundaries of land with wilderness character in the various parks would be difficult. Three US wilderness complexes have inscribed circles constrained by the border with Canada, and a glance at aerial maps for the areas north of the Mount Baker and Pasyaten wilderness areas show what looks like roads and clear-cuts over the border. However, the Boundary Waters wilderness is next to the vast Quetico Provincial Park in Ontario, which appears to be mostly primeval. So the radius for Boundary Waters could be as high as 13.6 miles if allowed to go over the border."
