The Septuagint version of Jeremiah is about 1/8 (about 2700 words) shorter than the Masoretic. According to the relevant research the excessive text of the MT version is due to additions of several stereotypical and conventional expressions, finally damaging the original form of the prophecies that is full of meaning and superbly measured.
A cause of less serious differences is a translation’s expected liberty in order for the meaning to enjoy a natural expression in the new language, and in this case especially to answer the religious needs of the Jewish communities of the Hellenistic era. Because of this liberty the Septuagint version uses about 100 words that are not to be found in the MT.
Both the MT later augmentation of the original text and the translating liberties of the Septuagint creates a distance between the texts that still remains a source of great philological problems. The Septuagint version of Jeremiah is closer to the original size of the text. The excessive text of the Masoretic version cannot be considered genuine.
Jeremiah chapters in MT and the Septuagint