You can think of it as, "filler" projects. Like when a project is not urgent to the client, it would be fine if a professional doesn't have to take out time out of their busy higher paying schedule to do it. They can get on it whenever they find it convenient, or during low sales.
Think of extra fast deliveries. If faster deliveries cost more than usual, then on its opposite, super slow ones can be a little bit cheaper.
Yes, this seems the correct argument for the theory.