One of the questions that comes up often concerns recent family history, and how the DNA results showing a totally different result could be true.
Example: In our family, one of my cousins spent a lot of time and effort researching our history. We all know our roots go back to Georgia in the 1800's, and she was actually able to trace them back to South Carolina in the early 1700's -- no easy task because Sherman destroyed many Georgia courthouses (and all records therein) in his scorched-earth march to the sea in the Civil War.
The history of Georgia is that it was largely settled by English people released from debtor prisons and sent to America to work of their debts as farmers. And of course, our family is from that area of Georgia where they were. However, the cousin traced a possible line to Switzerland in the early 1700's via SC. So from our last name (probably misspelled by illiterate debtor farmers), we settled on probably English and/or Swiss/German heritage.
My DNA test (Living DNA) came back 100% British Isles -- ~71% English, 19% Scottish, 10% Irish. And zero mention of anything in the US -- where we know for a fact we've been for almost 300 years, maybe longer!
But when I do Matching, the vast majority of my matches come back to the US. The rest are UK, with a few Canadian, Australian, and this flag which I have not been able to identify.
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The reason given for this lack of recent ancestry is that they are not really looking for recent history. They trace ancient migrations, and they give the ancestry 5-10 centuries previously -- which for Americans, would be where our ancestors most likely immigrated from.