In studying the differences between Bible translations, it will help us to understand why some scribes inserted certain words or verses into a manuscript they were copying. Occasionally, a scribe would make notes throughout the text of a Greek manuscript, and these notes would be incorporated into the text by a subsequent scribe.
“Another form of error, more difficult to solve, grows out of the practice of writing explanatory notes in the margin. These marginal notes are somehow incorporated in the main body of material and thus become a part of the text. But it should be stressed that the New Testament manuscripts rarely exhibit this kind of error, and that when it does occur, our many textual witnesses keep us on the right course. 2. Intentional errors. Unintentional alterations in the manuscripts are many, but the vast majority of them are of little consequence….We ought not think these insertions were made by dishonest scribes who simply wanted to tamper with the text. Almost always the intention of the scribe is good and he wants only to “correct” what appears to be an error in the text. So if a word seems improperly spelled, or a Greek verb does not have the proper ending, or a form does not correspond with the classical idiom, then the scribe feels it is his duty to improve the text he is copying….Again, citing one or two examples may illustrate the point. In John 7: 39 the text literally reads, “for not yet was the Spirit.” Because this could be taken to mean that the Spirit was not in existence at that time, some manuscripts and versions add the word “given” for the phrase to read, “the Spirit was not yet given.”…In these cases the scribes, with all good intentions, mistakenly thought that their additions were necessary in order to bring about a better understanding of the text. A scribe especially might try to remove any difficulty in the texts of the Gospels. If he found a statement of Jesus in one Gospel similar to a statement in another, he might modify one to make it in perfect agreement with the other. This may explain a variant found in two verses of Matthew and Luke. The King James Version of Matthew 11: 19 reads, “But wisdom is justified of her children,” an exact parallel of Luke 7: 25. However, the more recent translations of Matthew have “works” instead of “children” in agreement with our earliest manuscript authorities. We surmise that at some early date “works” was changed to “children” by a copyist to bring the phrase in harmony with Luke’s Gospel. Thus we are practically certain that originally the two records of Jesus’ sayings were not the same. This, to be sure, is what one frequently finds in the Gospels, for in quoting Jesus, the Gospel writers often do not give his words verbatim.” (Neil R. Lightfoot, How We Got The Bible, 88-90 (Kindle Edition); Grand Rapids, Michigan; Baker Books)
So there were good and logical reasons for these scribal errors in the New Testament manuscripts. There was usually not some sinister agenda to add to or take away from the text of Scripture.