[sammyg2]

By design, waste gates will open partially at a lower pressure than it will take to get fully open.
The spring holds it closed. Air pressure overcomes the spring and opens it.
The spring, being a linear device, will compress a little when fighting a little air pressure and will open more with more air pressure. Exhaust pressure also mucks up the plans as it pushes up on the valve unless you have one of them fancy jobs where the valve is balanced to prevent this from happening.

Say you set it to control boost at 9 psi. It will probably start to open at around 6 psi, and be fully open at 8. If the waste gate or piping isn't big enough to pass all the exhaust gasses necessary, the boost will continue to creep up. The exhaust gets hotter and has more energy the longer you stay at full throttle, so that comes into play too.
The device I installed is also adjustable and has a spring loaded check valve that remains closed until a pre-determined pressure overcomes a ball seal and causes it to "pop" and allow the boost signal to hit the waste gate. It bleeds a very small amount of air when under partial boost. I guess that's what keeps it from doing the same thing as the waste gate.
That pop prevents the waste gate from partially opening before you want it to.
I was skeptical when I bought it and installed it, but the improvement is obvious. It does work on my engine, no doubt about it.