In page 17, the Watchtower (Study Edition, December 2015), says:
For example, at 2 Samuel 13:21, the Greek Septuagint contains the equivalent of the words: “But he would not hurt the feelings of Amnon his son, because he loved him, for he was his firstborn.” Earlier versions of the New World Translation did not include these words because they are not in the Masoretic text. However, the Dead Sea Scrolls do contain these words, which are now included in the 2013 revision.
Note that The Watchtower has not shown a good argument for adding those Septuagint's words. They didn't include them "because they are not in the Massoretic text". However, we know that the Massoretic text does not contain, for instance, some chapters of the book of Daniel, but they appear in the Septuagint. Why doesn't the GB add them now?. Furthermore, the Dead Sea Scrolls contain writings of the Apocrypha, which appear in the Septuagint. This fact has been used in favor to Catholic Bible, which contains some of the Apocrypha.