Ed, you are correct: The knob adjustment only corrects the indicated temp. If you set at 225, and consistently get, say, 220, you can adjust the indicator, not the high temp range of the rheostat. It is limited by design.
Now, If you are using the Auber wall-mounted probe, I would recommend bypassing the onboard controller completely. This is simple, and there is a sticky topic on this in the Auber section. If you take the analog "limiter" out of the equation, install the wall-mount probe, and run an autotune (per the instructions), you will have the most accurate smoker on the planet!