@Libri-Liberorum It's intentional. From the prologue of the first novel:
The dukes had achieved great things back when the kingdom was founded. Because of their history and because the first generation of each ducal house is connected to the royal family by marriage, we address them differently than other countries would. Take Duke Howard, for instance—the head of the Howard family that guards the lands north of the capital. He would customarily be called “His Grace” elsewhere, but we address him as “His Highness.” His sons and daughters are styled “Highness” as well.
The original plan had supposedly been to enfeoff the dukes as royal dukes, but they had objected that this was “irreverent.” The royal family refused to back down, however, so the ducal houses eventually kept “Highness” as a compromise. I’d heard the dukes were secretly still included in the royal line of succession, but who knew whether that was really true. It was all so confusing.
