Actually, the dual line weight designation is probably more accurate that a single line designation unless the manufacturer is using an objective measure like CSS. Most manufacturers design rods to accomodate the most likely line used by the average caster of that particular rod. It is a subjective determination. That is why you see people over and underling rods to match their casting style.

Line weight refers to the line, not the rod. The first 30 feet of a line of a certain weight will weigh in a certain range (even this is not exact). The rod is rated at the line weight that, in the opinion of the designer, the average caster would be casting on it. If I am an agressive caster aerializing a lot of line, I may underline the rod. If I am a finesse caster making short casts, I may overline it. The dual line designation at least recognizes the subjective nature of rating rods.