We have a tendency to "finish the story" in our heads as we receive news.
We are concluding based on what we have experienced to be.
When in 1988 a huge "Birthday Party" was held at Wembley Stadium for then 70-year-old imprisoned Nelson Mandela, the world expected him to die in prison. Apartheid's end was nowhere in sight at the time and many already wrote him off as dead in their minds.
That is the Mandela Effect in a nutshell. It is not a unique phenomenon.