It's all poor editing about his parents, but all modules and supplements, including the 5e material, refer to Strahd as "Count." Per AD&D Domains of Dread, this appears by choice (describing Strahd as choosing not to rule like a prince or king).
So here's the inconsistencies about his parents:
First, there may be confusion about Prime Material Barovia and Dark Domain Barovia. Per the "Roots of Evil" AD&D module, there is a modern day Barovia and whatever exists in the Shadowfell is a carbon copy. They have Castle Ravenloft and the village of Vallki, etc., but Vallaki, for example, is an unwalled peaceful "resort" town and the Prime Material castle is quite nice. There are a lot of parallels, despite the centuries, such as both have a Blue Water Inn, an Arasek stockyard, and a Father Lucian. The modern Barovians believe Strahd is a hero who vanished when a dark fog settled on the castle and took the wedding party, including most of the royal family (except Strahd's brother Sturm, who wasn't there). The Domain of Dread AD&D supplement describes people who left the wedding early but were still grabbed by the mists (e.g. the Dilisnya family, which has a century long grudge with the von Zarovich line).
The Ravenloft Black Box AD&D set and Gazetteer create a timeline for Barovia and its history. Using a Barovian calendar (BC), Barovia was established Year 1 on an "outlander world" of unknown name. The Gazetteer describes it as an alliance of "independent city states."
299 BC, Strahd is born to Count Barov von Zarovich and Countess Ravenia van Roeyen. (per the Gazeteer). This conflicts with I6, the AD&D module that started it all, and the AD&D 2e reprint House of Strahd, which both call Strahd's parents "King" and "Queen" and spell the queen's name as Ravenonia. The first Ravenloft novel has a scene where Jander Sunstar is looking at a portrait of Strahd's parents, listed as "Barov and Ravenonia von Zarovich." And you've got "King's" rooms in the castle, and the burial vaults in every incarnation of Castle Ravenloft that have "Kings" buried there.
346 BC, his parents are killed by Terg invaders, and he takes over the army.
351 BC, Strahd's actions draws the land of Barovia into the mists (but as "Roots of Evil" and the nature of the Shadowfell show, it's people that were taken, everything else was copied).
740 BC, the Grand Conjunction occurs (big event where the Dark domains of Ravenloft spilled into the Prime Material plane). At this time, there is a Barovia Kingdom on the prime material plane ruled by King Barov von Zarovich VI and Queen Kristiana von Zarovich. The AD&D module "Roots of Evil" covers this. Not all the von Zarovich line were captured by the mists.
Conclusion: Strahd's parents were a King and Queen and the Gazetteer is an outlier error. Despite conquering his homeland after his parents' deaths, he never elevated himself to the title of King, likely due to his military heritage and lack of desire for the day-to-day management of a King. Back on Prime Material Barovia, they continued the practice of having a King and Queen using the von Zarovich lineage.