Well, it is not uncommon in anglophone practice to use terminology to describe titles and name places that were not used by the cultures in question. Both the Polish-Lithuanian Commonwealth and Byzantine Empire are two examples of this. The use of Tsar partially falls within this trend.

The change in terminology began with the reign of Peter the Great. Peter I accepted the title All-Russian Imperator in 1721 and it became the first title used by subsequent Romanovs. This was part of Peter I's efforts at building up his prestige and self-styling as a modern European monarch who had no peers. The tsar title implied he was still Muscovite, but imperator is a more ambitious.Yet the title of tsar still remained in use and became part of the various monarchical appendages that signified areas over which the tsar had suzerainty, such as the Tsar of Kazan, Tsar of Siberia, etc. The title Tsar also entered into the vernacular as a shorthand for the Russian ruler and subsequent emperors after Peter I sometimes preferred Tsar over Emperor. The archconservative Alexander III emphasized his older Muscovite title over the Petrine one. Here Tsar came easier as a form of address for a Russian monarchy increasingly reliant upon Russian history and tradition to legitimize itself.

In general, popular descriptions of Russian rulers in the nineteenth century used Emperor and Tsar interchangeably. Official etiquette stressed the use of Emperor. Historians of Russia stylistically tend to use Emperor, but use tsar as an adjective (e.g. tsarist Russia, tsarist police state) or a shorthand for the head of state.