Hint: the map is a plane... the world is a sphere...
The miles near the poles are "stretched" in the plane.
So the map actually has a bug because it's using several different scales all at once (i.e, the Mercator style of projection)... but the bug is the Mercator map itself, not the CN map.
[img]http://www.geog.ucsb.edu/~kclarke/G176B/proj1.jpg[/img]
Squaring the circle ain't easy!