There is also the subject of a writer's agenda.
Suppose there were a Kennedy in the 1950s or 1960s who was in a racing accident at a car race. Suppose also there were four newspaper reporters at the race. One of the reporters was a sports reporter, one a society reporter, one a political reporter, and one a general news reporter. One would expect that our four hypothetical reporters would turn in articles that were quite different even though they were all reporting on the same event.
Matthew, Mark, Luke, and John had different agendas. Matthew who was also called Levi focused his book primarily on the conflict between rabbinical teachings and Jesus's analysis of the Law given through Moses. He also focuses a lot of attention on Jesus as the Messiah prophecied in the the Tanakh. Mark focused primarily on action, his favorite word is translated in many translations as "immediately." He also focuses on the attitude of Christ of being a servant. Luke focuses primarily on the doctrines which Jesus taught and on his humanity. John does not even attempt to give us a in depth reporting of the the 3 years in which Jesus preached. He focuses instead on the difference between the physical and the spiritual. The emphais in John is on the spiritual implications of Christ's teaching and on the need for complete commitment to Him.
These four agendas cause many of the events regarding which they report to be presented from different "angles". For example, John is the only one who discusses Christ's refusal to feed those who came looking for Him, the day after He had fed the 5000 because they were motivated by the physicl and not by the spiritual.