From the Ded Sea Scrolls it turns out that at least some of the Septuagint comes from a different textual tradition than the Masoretic texts. A close match for the Septuagint Jeremaiah was discovered at Qumran. Other books contain readings which agree with the Septuagint or with the Samaritan Pentateuch rather than the Masoretic text.
While not all the Septuagint translations are good they do offer a limited window on the variations that were in the Jewish scriptures at the time of writing.