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That's a hard question to be honest. The reason this happens is that many fully ethnic members of a some populations show small percentages of other ancestries in their results. In order to make the results seem "more realistic" at a population level, the testing companies may release those smoothing updates. For example if you're 7/8 Polish and 1/8 German you would probably by the ancient admixture be very close to Slovaks because they carry that 15-20% Ostsiedlung Germans genes or ancient Celto-Germanic admixture. In order for Slovaks to get realistic results those companies have references for them that they consider to be 100% EE, so when the person with distant 1/8 Central European heritage tests he gets 100% EE because he is the closest to that Slovak reference. While this improves the aggregate results for people with primarily one ethnicity, it makes the results less precise for those who genuinely have distant minor ancestries. Their small percentages may get smoothed away in the process. I think it works like that but I thought that 23andme and AncestryDNA used a different method for calculating Ancestry Composition results.
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