Similarly, in the ANH novelization, Vader is one of many Sith, who are as much a servant of the Empire as the Jedi were for the Republic. Palps isn't even a Sith - just a politician who actually became overwhelmed and controlled by the very Empire that he was put in charge of, and became little more than a puppet.
Of course, all this changed much later, but the effects of these remnants remain, thus the questions of relative power between Tarkin and Vader.