One of the key factors with modern diesel engines is the injection timing, which is essentially the same as ignition timing on a petrol engine.

To avoid NOX values falling outside of the parameters they are allowed within when selling a new car the manufacturers often have to retard the injector timing to stop the diesel burning fully as NOx emissions tend to happen in the later stages of burn.

By advancing the timing back to where the manufacturer would have liked to have put it if their hands werent tied by legislation, it can normally improve both power and economy without a need to raise boost pressure.

Some remaps then also introduce a little more boost as well, but typically most decent remaps I have seen dont actually increase the boost by a huge amount like we all tend to want to do with our petrol turbos.