There were differences in translation between the Septuagint version and the Hebrew text. Once such difference is in the translation of Jeremiah, Chapter 25.
In the Hebrew text, Judah is listed as only one of a number of Middle Eastern countries in servitude to Babylon. According to the Septuagint version, however, you would get the idea that Judah alone was placed in servitude - thus helping the WTS stand that the "seventy years" meant something other than just mere servitude.
Bill.