The percentage of adherents figures look bogus to me. Given for example that about 12%-13% or something of the population is black, then that 1.6% for Black Protestants means that only about 15% or so of the blacks are Protestants. Really? The Catholic percentage of 38% or so also looks way high. It might be closer to 25%. Maybe the issue is that "adherents" really means "congregants," and those who claim an affiliation, may not be members of any congregation, and far more Catholics tend to be congregants than blacks. Or something.
BK explained one problem with the methodology, but another is that many churches don't have a formal membership, or they do but people are reluctant to join. Interesting and kind of funny article on this sort of thing with one church here:
http://www.christiancentury.org/article/2011-05/dismembership-planI find the band reference in the title very amusing, just because it's not something I'd ever expect to see a reference to in the Christian Century.
Keep in mind by the way the Black Protestant bit refers only to blacks that go to specific "black Protestant" churches, blacks that go to churches that aren't predominately black are counted as just evangelical or mainline Protestant.