Why do atheists love to inflate their numbers and make reference to the U.K and Sweden when we all know?

As opposed to Jews who consider everyone of Jewish ancestry Jewish? Or the Catholic church who says everyone that went to church ever in their entire lives, even if it was their own Christening is Christian? The UK is a secular, predominantly atheist country. But even if it weren't, what do numbers have to do with anything? If everyone but you believed in flying elephants, would that make it true?