THE line (top, in red) is supposed to be level. It’s created between two points, but in the Right view above, the left end is a stubborn 10 units above the level of the right end. The black line drawn in Rhino (not GH) underneath is level; I put it there more easily to show the unwanted slope of the GH line.
Thus, the question is, how to get the GH-line level and where did those extra 10 units come from? Currently a mystery!