The type of figures that auto manufacturers of any kind like to give are those when the car/engine is running in perfect conditions.
Lets be honest here, in terms of a power figure you're never going to be accurate because at the times when the power of an engine is recorded it's generally under dyno conditions or on a test bed, which is just about as far from a real engine application as you can get.
So many variables alter the power of an engine, altitude, humidity, temperature.. and that's not JUST for turbocharged engines of course, it's the same for a tractor even.