Setting up a specific demand for a specific line an isolated way is not realistic. Not realistic in terms of manpower. There are 2800+ airports in that game, that's around 4 million routes. Manually setting up demand for a specific line is not realistic for such a level of demand. PLUS, there is, in the works, a city-based demand system that is gonna replace the current engine.
So it's not worth even correcting it right now. It would mean a lot of work, for one line out of 4 million, and for a system that will be replaced soon anyways. That, plus what Sami said : demand is a complex beast, with plenty of parameters.