If you juxtapose two constant-width curves stroked along an identically-shaped path side-by-side, they don't necessarily fit tightly together. At left, there's are a red curve and blue curve both made from four circle quadrants, where the radius of the circle is the same as the width of the stroke. If the width of the stroked line, and the radius along the center of the line, is 20, then the radius along the outside edge of the line is 30 (radius plus half line-width) and the radius along the inside edge of the line is 10 (radius minus half line-width). A circular arc with radius 30 cannot fit inside a circular arc with radius 10, so the two lines do not fit tightly together, as seen.