I think part of the case here is that people are applying gasoline engine physics to a diesel engine. For one, the parasitic losses between the rpms of 5th gear and 6th gear in my truck is minimal. Diesels have significantly less pumping losses versus a gas engine. Diesel is not like a gasoline engine that has to stay around a 14.7:1 air/fuel mixture. With a gas engine, the more air added due to higher rpms pumping more air through a gas engine, the more fuel that needs to be added to keep from going too lean.
Diesels don't operate that way. They are regulated by fuel and can lean out utilizing more air and less fuel. Most modern diesels operate from 20:1 to as high as 60:1 air fuel ratio depending many variables. Especially where the turbo compressor map is most efficient meaning it is utilizing more air per drop of fuel to make power. At low rpms, depending on your turbo, your engine may not be able to spool enough air for the amount of fuel being injected therefor it's air/fuel ratio is on the rich side.
This is why EGT's climb much higher at low rpms because turbo cannot spin fast enough so the engine does not have enough air along with less air from lower rpms for the amount of fuel being injected to make the amount of power that is demanded from the driver. Basically, mid-high load at lower rpm below the compressors efficiency range means more fuel is needed to make power. At mid-high load at higher rpms around the compressors efficiency range, the engine needs less fuel to make power since it has more air available and can lean out it's air/fuel ratio to make power.
A diesel will just utilize the amount of air available for the amount of fuel being injected. Hence the reason why my engine is under more of a load in 6th than 5th when towing my trailer.
