Rowling has a reccurring hangup with changing the past. In the late sequel screenplay thing that stars Harry's kid, the plot is that they want to go back in time to save Cedric Diggory's life, and succeed - but somehow this makes Cedric himself become a Death Eater and causes the bad guys to win the wizard war, so they end up having to make sure Cedric dies as originally planned.

It's "we live in the best possible reality".