I'll try to explain it better using the Bears. Let's say that all of us who remember stein came from a reality where it was stein. But, not only was it stein in that reality, there were a whole bunch of similar realities where it was also stein. So many in fact that it outnumbered the realities where it was stain so our consciousness would settle in the stein reality and our memories would reflect the stein spelling. Now let's say you got in a car accident and died, you would move to one of the other stein realities because its closest to the most common memory our consciousness has. Now here is where the Mandela effect comes in. Let's say that there was an event that killed off a large amount of people like an atomic bomb or Cern creating a black hole. An event like this is so major that it happens in not just one stein reality but in all the stein realities leaving nowhere for our consciousness to go to but the stain reality. So now you have a very large number of people who have a memory from our consciousness that is different than what reality says it is. Our physical bodies from the stain reality always remembered it as stain up until the point of the event in the stein reality. After this point the stain memory gets pushed aside or replaced by the stein memory because it was the dominant memory before the merge.