mr_psi
Sounds simple to me...you have a manual boost controller that provides an offset or additional boost value to the oem boost, not a fixed value.

The standard Rex ECU runs a falling boost curve. Meaning at higher revs it drops boost. Because if you ask a TD04 to push 1.3 bar at 6000 rpm its going to be an ugly (hot) result. They normally taper to about 0.6 / 0.7 bar at redline for saftey.

Your MBC valve would be set to trick the system to run say 0.1 or 0.2 bar higher than original boost values across the entire rev range. Have a look at some dyno plots of standard cars on RPM/Boost if this explanation is not clear.