I'm tackling one layer at a time here. Population B is indeed a sample of A, and its representativeness is one of the base questions. That means I need to expand the "units" of the denominators. So let's try to translate those units: The census data remains population A, although we know it is not 100% accurate. This has been an issue for decades with the census and Congress has had a tizzy fit whenever the Census Bureau has suggested using statistical techniques to try to correct it. The implication here is that, even population A has a +/- to it. We know, of course, this is true of the sample of A that I had referred to as population B. This also has a +/- to it, and a larger one at that.
With that in mind, the denominators are actually "population A +/-X" and "population A +/-Y", where Y > X. We can do similar things with the standard deviation, kurtosis and skew. Any way we slice it, though, those denominators are not truly the same, and it is the sameness of those denominators that is the original question.