[dcb]

The few bmep maps of diesels I have seen put the peak power to fuel in the 2-3k rpm range with full torque(throttle). So even though it isn't fast acceleration, you are accelerating more efficiently getting the most power out of the fuel, and still not using as much fuel because the engine is turning slower and squirting less often (even though it is squirting a bit more per revolution).

Andyway, that's my guess.