Its quite normal - manipulating the propeller pitch will increase/decrease engine RPM, and the engine RPM is in turn what defines the supercharger RPM. The faster the supercharger runs, of course, the more air it delivers to the engine, and it can mean that you can get a bit of extra power above the rated altitude of the engine, as it can maintain boost to higher altitudes.
Below rated altitude, it should not matter or even decrease engine power somewhat, since the engine is throttled (it wont get more than normal boost)
IIRC the DB 601 had something like an 1:10 gear ratio between the engine and the supercharger, meaning that the supercharger revolved at around 24 000 rpm if the engine was revving at 2400.) In November 1940 for example the LW even issued a circular that sanctioned this - apparently already existent - practice. See:
http://kurfurst.org/Engine/Boostclea..._increase.html
Can you post this part from Steinhilper perhaps?