Why are National Baptist hymnals not coming up as Baptist?

When I search the database for Baptist Hymnals (using the Advanced Search form), I get a couple hundred hits, and while they include pretty much everything I can think of as far as SBC and ABC/NorthernBC books go, and several Swedish Baptist and German Baptist (in the USA sense, not the languages) and Primitive Baptist hymnals, as well as sporadic "other" ones (IFB, Kenyan, Cuban, Canadian, Russian, British etc.), none of the recent National Baptist Convention USA books come up, i.e. New National Baptist Hymnal (1977), New National Baptist Hymnal 21st Century edition (2001), and Total Praise (2011), though they *do* come up if I search for "National Baptist" instead of "Baptist" as the Denomination on the search form. Older NBC of America books (ending in 1924, I think) do come up as "Baptist" (as well as as "National Baptist"). I am guessing some of this is explained by the not always activated "Resolved Denomination" bit, which I'm not sure I understand.