In a nutshell it's basically what DH1 said about peak ratings. This is from another site that (I think) descibes it well:
You have to consider how horsepower is rated in order to compare diesel engines to gasoline engines in a typical garden tractor. The horsepower ratings that you see are peak horsepower ratings. Gasoline powered engines in garden tractors typically have peak horsepower at about 3000 rpm and horsepower falls off quickly below that . Diesel engines in compact tractors typically have their peak horsepower at about 1000-1500 rpm and horsepower stays up throughout most of the rpm range. How does that make a difference? A gasoline powered garden tractor has to be at full throttle to achieve its rated horsepower. When the tractor has a load put on it by a mower or going up a hill, for example, the rpm will fall from, let's say, around 3000 rpm down to 2500 rpm. At 2500 rpm a gasoline engine will have significantly less horsepower than at 3000 rpm. At 2500 rpm, with much less horsepower, the load will pull the engine down even further until the rpm and horsepower are so low that either the engine stalls or you release the load (by shutting off the mower or pushing in on the clutch) A diesel engine, however, has a much broader power band. Using the same example, the diesel powered tractor running at full throttle encounters the same load as the garden tractor did. The engine rpm will fall from, let's say, 2000 RPM to 1500 rpm. At 1500 rpm the diesel engine will have roughly the same power as it did at 3000 RPM and will continue to pull the load. The horsepower and rpm will not continue to fall because the diesel engines power is not as dependant on the engine's rpm.
Actually,.If you look at kubota engine specs, They have there highest hp rating @ there highest rpm, The 482 peak is 3600 rpm, the highest torque peak @ 2600 rpm Eric