I believe that is a mistake. This particular monarch is called 'Jan III Sobieski', indicating that he was the third Polish king whose regal name was Jan (Polish equivalent to English 'John') after Jan I Olbracht (reigned 1492-1501) and Jan II Kazimierz (r. 1648-1668) with 'Sobieski' being his actual last name. Although usage of the last names might seem unusual in hereditary monarchies (it was generally redundant, after all), this custom became quite popular in Poland after the adoption of electional monarchy, where particular monarchs did not form a dynasty and were freely elected from various candidates, both foreign and domestic, leading to usage of their family names (de Valois, Bathory, Vasa, Wiśniowiecki, Sobieski, Leszczyński, Poniatowski) in place of monikers that were commonly assigned to kings from Piast and Jagiellon dynasties. Two kings from Saxony-Anhalt Wettin family are a notable exception, as they are commonly known in Polish historiography as August II the Strong (r. 1697-1706 and 1709-1733) and August III Saxon (r. 1733-1763).