It might be the glorified aquarium valve.
If it’s tee’d into the stock boost solenoid or tee’d into a hose running from the compressor housing to the wastegate actuator, opening the valve will bleed boost to atmosphere.
With it closed, the wastegate actuator opens at 15 psi boost so that’s what the engine sees.
When you open the valve to let 5 psi boost bleed to atmosphere, the wastegate actuator still opens at 15 psi boost.
The engine sees the 15 psi boost from the actuator plus the 5 psi boost going to atmosphere for a grand total of 20 psi boost.
If you could post some photos of how it’s plumbed, that should help determine what type of boost controller you have.