No fuel is injected on overun, the engine is turned by the kinetic energy of the vehicle. The engine doesn't suddenly turn friction free because its using no fuel! It doesn't matter what is turning the engine, it uses more energy to turn it at 1600rpm than it does to turn it at 800rpm. I've also ignored the fact that when there is torque through the gearbox during overun 'engine braking' there is increased frictional losses too.
Essentially it uses more fuel to engine brake and then accelerate to regain the lost kinetic energy than it does the coast on idle. Physics
