Sounds to me like lazy, sloppy research and/or editing by the writing staff.
Perhaps some of those numbers are of Germans who died in concentration camps, some numbers are of JWs of any nationality who died in a German-run camp, maybe they throw in some numbers of ones who died in a prison which was not a concentration camp, etc.
One writer in 1974 looks up info from one source, while a different writer in 1980 looks up info from a different source, and whoever edits the stuff is too inept to look up the correct numbers himself, so he just assumes it.s right.
Given the lazy/sloppy research in most of their journals, it wouldn't be a big surprise if that explains most of the discrepancies.