Because those were a mess IRL to begin with in so many cases, and varied from country to country.George proved to be remarkably unwilling to flesh out the nobility system for Westeros
Personally I think its best to assume hes using a variation of the late medieval-early renaissance nobility system from Poland [all nobles are equal by law]. In this case, all minor nobility holds equal rank so long as they have land, and are differentiated by the amount of vassals [and those vassals vassals]. Higher status nobility is determined by ancient titles or offices held [granted by king]
In the classic western noble system it would be King- Duke/Count- knight houses
[The king could have his own vassal knights, a duke could have counts and knights. Simplification! Eat with ice cream!]
High Lords of the Dreadfort, Lords over the Lonely Hills, the Long Lake and the Weeping Water, Masters over the Houses Shiver, Weeper, Burley, Overton, etc....
With how big the north is, how much land Boltons have, and how many levies and knights they can field wouldn't that make Rose a Duke?
In fact, with those lands being a kingdom in the past, each of the mentioned lands [Lonely Hills, Weeping Water, Long Lake] could be their own ducal titles, with numerous fiefs and counties.