my reasoning is that if you draw abrupt lines at the thinnest point of a shape, it makes that bit seem small and the bits around it seem large by comparison (see belts on hourglass shapes). if i draw attention to my comparatively small ankles, it looks like my calf just balloons out above it.
it would be a very bad thing, too, if the line ended mid-calf - as lines at the widest points on a shape make the shape look wider at that point. visually speaking, to look longer and slimmer, the best thing is to have no lines at all (regular tights plus matching footwear / bare legs and flesh toned footwear), or as a compromise, to make sure that horizontal 'lines' caused by changes of colour or fabric hit on neither the widest nor the smallest points.
it's why 3/4 length sleeves are better than wrist length ones to make your arms look slimmer.