The World Prefers Simplers Explanations
Poirot assembles all the suspects and presents two possible scenarios. One is a simpler scenario; the other is a more complex one. Between these two scenarios, Poirot decides to present the first, more simple one to the Yugoslav police at the next train stop. The second, more complex story, Poirot decides to keep it as a secret shared between the twelve passengers and himself.
The world prefers simpler explanations. The world will not understand the complicated reasons and stories of people's pain, anguish and need for justice.
Stories involving people are always complex. Never are they so simple. Yet the world prefers simpler stories and simpler conclusions. So the true nature of human incidents may be better to be hidden as a secret for those directly involved, and simpler versions be given out to the public. One cannot judge things by outward appearances. One is not qualified to judge until he hears the inner story.