You are still confusing Ethnicity estimates and Ancestry Communities Bridget.
Ethnicity estimates are probability ranges of a link to a particular ethnicity region around 500 years ago or greater. They are determined by DNA analysis of segments that show consistency with Ancestry's reference panel of individuals that they have allocated to that region. As Ruskie says, many of the reference panels consist of very few individuals, so the possibility for error can be large (hence to some degree the range of probability which the single "headline" percentage does not express clearly).
But the communities are a completely different association, derived in a totally different way, as I explained in our PMs, and which is also explained in the link that Ruskie quoted in her post. The only DNA link to communities is that autosomal DNA matches are used to identify individuals who are related by family and/or who have shared matches with others. The public family trees of the resulting set of individuals are then trawled by Ancestry for particular locations (or communities) that are common to those individuals at specific points in time within the last 50 to 300 years.
So the communities aren't using DNA to predict anything, other than to select the group of individuals whose trees are searched for commonality of locations. The results rely heavily on the facts recorded in related public family trees.
Ethnicity estimations are based on identification of DNA segments that Ancestry believes identify populations that would have been in common to a specific region more than 500 years ago. So they are based on DNA. But Scottish ethnicity doesn't necessarily mean that ancestors of people attributed with that ethnicity actually lived in Scotland. Although Ancestry have chosen to give it that name, it identifies lengths of DNA that they believe are common to many people now found in Scotland and other areas of the UK with what might be better described as Celtic origins. Those people migrated to the British Isles from the European continent via various routes. Although many of them settled in Scotland and the other regions that Ancestry's Scottish ethnicity encompasses, some settled in other parts of Britain and never went anywhere near Scotland. So it is quite possible to have Scottish ethnicity as defined by Ancestry, without any direct ancestral lineage to Scotland itself.
My paternal father's family have around 37% Scottish ethnicity according to Ancestry, with no evidence of any connection to Scotland the last 350 years (other than a single Scot who married into the family) and indications that they probably arrived in Britain from NW France several hundred years earlier.
When you say that allocated communities can't be based on where people say they are from, because the individuals you know aren't aware of the connection and it isn't in their trees; it doesn't have to be. Much of the location information would have been obtained from other public trees of individuals that have autosomal DNA matched to "your" individuals, and the trees of shared matches between them.
Which brings me back to the USA settler communities we discussed, that my brother and I are, according to Ancestry, a part of. But of which our only connection to is through the half cousins descended from our GGF and a woman with whom he fathered children before he married our GGM - so no direct line of descent between them and us at all.
