Greek literary conventions permeated most Jewish literature written in the Greek language, and were applied both to historical books (which the Gospels claim to be) and novels alike. Writers of topical biography had complete freedom to rearrange their sources, so it should not surprise us that Matthew and Mark have many events in Jesus’ ministry in different order. Although Jesus, like other Jewish teachers, surely repeated the same sayings on separate occasions, some of his sayings probably occur in different places in the Gospels simply because the writers were exercising the freedom ancient biographers had to rearrange their material. This freedom enabled the Gospel writers, like preachers today, to preach Jesus as well as report about him, while still recounting his words and deeds accurately. Ancient Christians already knew, of course, that the Gospels were not in chronological order, as the early Christian teacher Papias plainly remarked about Mark.
Craig S. Keener and InterVarsity Press, The IVP Bible Background Commentary : New Testament (Downers Grove, Ill.: InterVarsity Press, 1993).
What does this mean? At the very least it means the differences in sequence may not be used to question the inerrancy, authorship, inspiration, etc. of the gospels. The gospels must be understood according to the literary norms during the times of the authors. If it is true that all four gospels are topical biographies, then differences in sequence are merely a means of emphasis to communicate the intended meaning.
So, when there are differences the question that must be answered is what point the author was intending to make. There are two tools used in the exegesis of the gospels: vertical analysis and horizontal analysis. In vertical analysis, the exegete does a careful reading of the larger context of the passage, searching for repetitions and themes. In horizontal analysis, the exegete does a careful reading of the parallel passages in all four gospels, looking for similarities and differences. An important clue to the meaning is the intersection of the emphases in the vertical and horizontal analyses. This technique respects the uniqueness and integrity of the individual gospel and the relationship between the four gospels.
Bill Arnold is discussing this topic from a different perspective here.