Or it could be a scribe added Matthews' record to Marks.<quoted text>
It is interesting to note that you chose to cite the later the later text.
The section in modern Mark that is taken as the "Great Commission", does not exist in the oldest extant Koine copies of the text: the "Codex Vaticanus Graecus 1209" and the "Codex Sinaiticus".
This suggests that the section you cite from Matthew, the later text, is a later addition.
However, I could have used the baptism of Jesus too...