My fort has no holdings, tributaries, vassals, of any kind.
Yet I've been "blessed" with 3 barons and 1 count in less than 3 years of play in un-modded 0.47.05
Unless I'm reading the wiki wrong, it says I shouldn't have any barons or counts.
Were they appointed (dialogue box with outpost liaison) or are they just migrants/visitors who are/became barons and counts of other places. They'll still order people around, and the migrants can't leave to go order around their own people of course in the current version.
Dwarven nobles have no apparent duties or responsibilities. This leaves them free to wander and migrate to your fortress, but they will never leave. They still expect to have their mandates, demands, and requirements met. It seems that some worlds and civilizations cause this more than others as some fortresses never see this while I have had others where as many as 6 nobles migrate to my fort.
Also, sometimes the heir migrates to your fort and then inherits the title.