A Gov 6 world can also be ruled directly by an adjacent world, or organisation, not just the central Imperium government/Capital.
In fact, most Government 6 worlds have (at least in MT, IIRC also in CT) the note owned by xxxx, xxxx being the name of the planet under whose domination is. In the case of Terra, CT and MT made it quite clear it was under Imperial military rule.
In MT:HT, many code 6 goverment worlds became military rule, as the miliary takes the power under that crisis (martial law), and the crisis lasting longer than expected, I guess most of them developed to A or B governments (dictatorships). Anyway, talking about Imperial Nobles in HT (and even in MT), where Lucan has so meddled with noble patents and the moot may be quite confusing, to say the least...