A scribe is far more likely to omit a word or phrase mistakenly, as to intentionally adding. The reading that is deemed immediately at odds with the context is preferred if deemed intentional because a scribe is more likely to have smoothed the reading out. The harmonization of passages is likely an intentional change by a copyist, who is seeking to have a passage agree with a similar passage from another book. Examples: Doctrinal Corrections, Liturgical Corrections, Harmonistic Corrections, Historical Corrections, and Linguistic or Rhetorical Corrections.
NTTC MARK 1:2: Was the Original Reading “it has been written in Isaiah the prophet” OR “it has been written in the prophets”?
The quotation in verses 2 and 3 is composite, the first part being from Mal 3:1 and the second part from Is 40:3. It is easy to see, therefore, why copyists would have altered the words ...