We don't know everything. For all we know, if such a simulation ran there could be a overwhelming amount of A's that turn out as Y's, and visa versa.
In other words, as a dumbed-down example, if an egg balanced on a roof in our universe fell off the right side, what's to say that in the simulated universe something we do not know of or understand had changed the fate of the egg, causing it to fall off the left side? The universe and how it unravels is probably much more complicated than we could possibly ever understand. There is no way we could possibly ever know all of the factors that influence how we came to be, much less write it all in a program.