"Because realities do not travel at the same pace. The multiverse is thinner than you might wish to imagine. An event of sufficient magnitude - the defeat of the Great Vampires and their expulsion from the timeline - can cause...well, radiation might be the best term, for you. That reverberates through cracks and spots in the fabric of reality that are stretched thin. Thus, an event of sufficient magnitude in one reality can create reverberations in many - and any number of...mutations, shall we say."