Jamieson, Fausset and Brown on Jeremiah 25:13

13. all ... written in this book, which Jeremiah ... prophesied against all ... nations--It follows from this, that the prophecies against foreign nations (forty-sixth through fifty-first chapters) must have been already written. Hence the Septuagint inserts here those prophecies. But if they had followed immediately (Jr 25:13), there would have been no propriety in the observation in the verse. The very wording of the reference shows that they existed in some other part of the book, and not in the immediate context. It was in this very year, the fourth of Jehoiakim (Jr 36:1, 2), that Jeremiah was directed to write in a regular book for the first time all that he had prophesied against Judah and foreign "nations" from the beginning of his ministry. Probably, at a subsequent time, when he completed the whole work, including the forty-sixth through fifty-first chapters, Jeremiah himself inserted the clause, "all that is written in this book, which Jeremiah hath prophesied against all the nations." The prophecies in question may have been repeated, as others in Jeremiah, more than once; so in the original smaller collection they may have stood in an earlier position; and, in the fuller subsequent collection, in their later and present position.