Then why does it sound like you're entering a single static value for duty to control boost ?

You will almost always need to vary duty with load to achieve a steady boost figure throughout the rpm range, especially if there are inefficiencies within the setup.
Which at a minimum if it isnt closed loop control, will need to be boost vs rpm

Map the boost properly and you will achieve a far steadier figure.

But as said, there are many many variables from the engine itself, turbo, actuator, control methods, possible faults etc etc that can affect the boost you see.