MyHeritage shows you many matches that occur in well-documented excess-IBD (“pile-up”) regions (where far more matches appear than would be predicted by statistical models), and the vast majority of those matches are not related (except insofar as Scandinavians are very distantly related to, say, English people). At the same time, though, people who actually are related in a genealogical sense may share one or more of those segments as well, so while those segments are not useful for determining whether a relationship exists between two people, they may (if they are large enough) be important for determining how closely two people already shown to be related are actually related. The reason this is true is that some of the excess-IBD regions are quite large, and some people may match random people on 30-cM segments there, but also match their own known relatives on those segments, so filtering out those segments may cause the closeness of those relationships to be underestimated.
Ancestry will try to help you by filtering many of the “false” matches out for you, while MyHeritage will show you many false matches who are unrelated to you (in a genealogical sense), but give you the critical genetic information about your matches, which Ancestry withholds from you. One good clue that you may be looking at a “false” or excess-IBD match on MyHeritage is if you see numerous other matches in the shared-matches tab with triangulation icons next to them, and when you click on the triangulation information for those matches, you see that they are all matching you and each other on the same segment, in spite of having very disparate ancestry that suggests they are unlikely to be related to each other. The chances that you will have more than a few relatives (apart from very close ones) triangulating on the same segment is very small, and far more likely to be a sign that that segment is not genealogically meaningful (because it is located on an excess-IBD pile-up region where enormous numbers of people match each other, even if that has yet to be documented).
MyHeritage will also include even very small segments in the total segments/cM counts, which will inflate the totals for some matches, so the MyHeritage totals are really an upper bound rather than a precise total (which is the opposite for Ancestry, where the totals are more likely to be a lower bound, since some segments may have been filtered out). To get a better picture of this, you really have to look at the same matches on both sites, because MyHeritage gives you a lot of misleading matches, but provides the key data, whereas Ancestry hides some of the most important pieces of information about your matches (which location on which chromosome you match, and who else matches you at that location), but does a much better job of presenting you with genealogically useful matches (because when you are seeing only a tiny fraction of your matches anyway, it is much more efficient to focus on the ones most likely to be genealogically useful). In any case, there are many matches that will only appear on a single site, so it’s worth looking on all of the large sites.
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