Start with the realization that each Gospel has its own author, intended audience and purpose. Each author tells Jesus' story from his own viewpoint, selecting and recounting events and sayings in Jesus' life to make his points and convey the lessons he wishes.
Further, the Gospels were written after the death and resurrection of Christ. The evangelist told the story of Jesus in the light of post-resurrection events and faith. He described past events with knowledge and understanding that came from the present or future.