Fringe presents a wonderful concept of existence of a parallel universe. In one episode, Walter explains how the choices made by the people affect the lives in parallel universes.
Of course, with time a lot of choices are made and they affect our lives.

Both the universes are shown with the same set of characters with a little difference in their lifestyle.

Now my question is, How come the same versions of people exist in both the universes, when every single choice made, affect the course of the future. If every choice affect the future, then why not some characters did not exist in either of the universe. I know the father of Astrid died in one of the universe, but my question is some of the characters must not have born in either of the universe.

I know from the tv-series point of view, it might not have been possible to introduce new characters but the concept could have been more realistic if that would have happened.