Loft surface is generated by using multiple 3D curves or existing CAD edges. When a loft surface is generated, iso-lines are also constructed on it according to the number of control points in the selected entities. However, if each selected curve has different number of control points or if they are of different shapes then the iso-lines on the resulting loft surface will be irregular. A surface with irregular iso-lines may cause some problem in the manufacturing process. Therefore if you want to get a high quality surface, you need to arrange the iso-lines.