TL;DR magic is involved in maintaining the country, at least for the Zent (= the king), the Aubs and the temple bishops (not the case for Giebes or head priests). They kind of deserve alternative titles in that sense. Citizenship is also tied to magic.
Ehrenfest is actually bottom of the barrel in Yurgenschmidt (the name of the country), and while national politics are effectively influenced by a handful of big duchies that have blood ties with the royal family, they still don't have a super-title like archduke, and Aub is equally used for everyone managing a region. So you can see the Aub as the equivalent of Duke or Archduke, but Sylvester is definitely not an Archduke if we were to use familiar titles.
Below the Aub are Giebes (Counts / Barons) who manage land subdivisions (like Brigitte's father or the Leisegang (the third wife of Karstedt comes from there and you will see that name pop up a lot since they are pretending Rozemyne is her biological daughter)), and other nobles that don't own any land don't have titles either (they can still be part of the family of a Giebe).
Being an Aub has a special meaning in terms of magic as well, they can do rituals and other actions that only work in their region. This administrative break down can only be modified by the Zent (which also has other special powers that work across the whole country, you can't just kill him and declare a new dynasty (but you can steal his title some other way), it's pretty much magic contracts all the way up to the gods), which is why in the first chapter, you see a map with "former Zauskas under Klassenburg management", the current Zent couldn't follow the inheritance procedures due to a civil war, and he cannot merge Zauskas into Klassenburg, more drama ensues.
However if you are looking for titles/ranks that have "arch" in the name, there is archnoble / mednoble / laynoble (and equivalent composites archknight / archscholar because they are extremely classists), which are ranked below the Aub and independent from the status of Giebe. Archnobles obviously have more political power and tend to have more blood ties with the Aub family.