Those disparate results for four siblings, as reported on the blog, do suggest that the quality of the ancestry data in the database is not good enough to support the level of precision implied by the conclusions that are reported.
23andMe provided very similar ancestries for my sister and me, and they are generally consistent with our understanding of our heritage. We are the two columns of percentages below:
|| Northwestern European
|| British & Irish
|| French & German
|| Broadly Northwestern European
|| Southern European
|| Broadly Southern European
|| Broadly European
||East Asian & Native American
I get confused when I supply the same data to the ancestry algorithms from Eurogenes, etc., on Gedmatch, as well as the algorithm used on DNA.land. For example, DNA.land says I'm 100% West Eurasian. I think that's a pretty solid conclusion, but their more detailed breakdown diverges pretty significantly from 23andMe and my known genealogy. They subdivided "West Eurasian" as 67% Northwest European (Scottish,British, Icelandic in Iceland, Norwegian, or Orcadian), 17% Northeast European (mostly North Slavic (Belarusian, Estonian, Lithuanian, Mordovian, Russian, or Ukrainian), 10% Mediterranean Islander (Cypriot, Sicilian, or Maltese), and 5.6% Ambiguous. In that analysis, the 67% Northwest European is what I would expect, but the omission of a German or French component and the large components of Slavic and Mediterranean Islander are totally out of line with what I think I know. It appears to me that the 23andMe ancestry is based on the distribution of peoples in Europe in the late middle ages, but the ancestries supplied by DNA.land, Eurogenes, etc., must dig farther back into the origins of the Vandals, Goths, Visigoths, etc., who populated Europe around the time of the fall of the Roman Empire.