You seem to be assuming that my High Nobles rule member worlds.

They do not.

Imperial tax fiefs are just that - areas of the Imperium from which Imperial taxes are collected.

An Imperial High Noble is not really comparable to a modern government official, at least IMTU . . .

[Trying to think of an appropriate analogy . . .]

One of my Imperial High Nobles is rather like an IRS District Supervisor who gets to spend 1/3 of the taxes he collects directly, while sending the rest upstream to Congress. Except that he is also in control of all of the FBI, US Marshalls Service, National Parks Service, Border Patrol, etc, etc, etc offices within his tax fief. But he is not a state governor, does not command the state police or the state National Guard (unless the latter has federalized in time of war), has no influence on state elections (except insofar as his public endorsement of one candidate versus another may have unofficial influence), etc.

Except in very rare cases - such as new colonies still under control of the Ministry of Colonization or ex-rebel worlds under control of the Imperial Army or Navy - my Imperium does not rule worlds directly. Some very powerful Imperial High Nobles - Sector Dukes and above (IMTU) - do control worlds directly, but in most cases this control stems from extra-Imperial positions. For example, the ruling house of a particular world was granted an Imperial title in order to incorporate them into the expanding Imperium back when.