Sweden is the most unusual when it comes to numbering monarchs with ordinals. Two names, Charles (Carl) and Eric both have fictionalized ordinals. Early Swedish monarchs were known by their full name. For example the first king Charles of Sweden was King Carl Sverkersson who first ruled the Swedish kingdom of Gothenland, and then king of a united Sweden from circa 1161 to his assassination in 1167. The next king to be named Carl is King Carl Knutsson who ruled Sweden from 1448-1457, 1464-1465 and from 1467 to his death in 1470. Carl Knutsson was also king of Norway 1449-1450. On his tomb Carl Knutsson is rightfully, and posthumously, called King Carl II of Sweden.
There would not be another king Carl of Sweden for nearly 200 years until 1604. This Carl read a book on Swedish history by Archbishop Johannes Magnus entitled Gothorvm sveonvmqve historia, which mentioned six previous kings of Sweden named Carl prior to the reign of Carl Sverkersson. In reality these six kings named Carl never existed for later historians were unable to find any source that verified that these kings lived. Regardless, the king named Carl who assumed the throne in 1604 called himself King Carl IX of Sweden based on the faulty information. This lead to the inaccurate numbering of the two previous monarchs named Carl as Carl VII and Carl VIII respectively. This numbering was never corrected and today King Carl XVI Gustav sits on the Swedish throne even though he is only the tenth king to actually be named Carl.
The reason Carl IX followed the faulty numbering of the Swedish kings may be due to the fact that his older brother, Eric, also followed the same incorrect information found in Johannes Magnus’s book. The older brother of Carl IX called himself Eric XIV of Sweden despite the fact that there were only six kings of Sweden named Eric that historians are certain of (there may have been more) prior to the reign of Eric XIV. King Erik Knutsson who ruled Sweden from 1208 to his death in 1216 is called Eric X although he was actually the second king Eric of Sweden that there is certainty about.