The name given for the king mentioned in the Greek Septuagint version of Esther is ArtaXerxes, who was probably ArtaXerxes I, king of Persia.
However, the Septuagint accounts of Ezra and Nehemiah give a different name for what appears to be the same king. There he is called ArthaSastha. Yet the Hebrew Masoretic texts of all those books consistently identify him as ArtaXerxes. Why is there a discrepancy in the Greek?
If all three accounts are truly speaking of the same ArtaXerxes I (which seems to be so), the difference may simply be caused by the Septuagint translators of Ezra and Nehemiah using a local Jewish spelling of the name, while the translator of Esther preferred the Greek spelling.