Perhaps the doubles milage limits are based on the fact they are near always on the big road, they have developed a milage average history that is very predictable and run dedicated routes.
Expediters, on the other hand, seldom know the routes they will travel, don't always stay on the big roads, don't often drop-and-hook-and-go, don't travel during the optimum times for maintaining the speed limit, and don't get to fuel at the pick-up point like FedEx Ground does.