Because they done some kind of yid math, and somewhere there's a Shlomo Levinstein who knows the exact ratio of immigrants to reduction in white population at year 10, or something - like, for every one non-white immigrant, the census value of white population as % of the total declines by X% with a confidence interval of Y%. Sure, there is an ideological bent to all of this as well, justified by some Jewish msyticism, but I would virtually guarantee you there is some secret equation. They're going by the numbers.