DNA Testing: What do ethnicity estimates really mean?
If you are one of the millions of people who have had their autosomal DNA (atDNA) tested through one of the major testing companies and have come away confused or questioning their validity, please know that you are not alone.
Many of the people that I talk to purchased their test believing that the testing company was going to tell them where their recent ancestors came from. Unfortunately, that's just not possible. What the tests are really telling you is that your DNA matches living people from these (fill-in-the-blank) countries. Period. That's it.
Don't get me wrong, ethnicity estimates are valuable, but they should be taken with a large grain of salt—perhaps even the whole shaker!
For example: One very common admixture that shows up in Western Europeans is often large amounts of Scandinavian and Iberian ancestry, which isn't reflected in the tester's family tree. How did those results get there? Was great grandpa really a Swede and not a Scot? Did great grandma have a secret love affair with the Portuguese sailor next door?
The answer is... most likely not. What you are seeing in your test results is a reflection of the past—the ancient past—and not where your great grandparents came from. As you may already know, the Vikings raided Scotland and Ireland for centuries, and even founded the town of Dublin!  Many of them settled in their adopted country, and that is why Scandinavian heritage shows up in people who consider themselves exclusively British, Irish and/or Scottish.
The same is true of Iberian seafarers who crossed the ocean from what is now modern day Portugal into the British Isles. Their descendants settled in Ireland and Scotland thousands of years ago, and your modern-day genetic makeup reflects these ancient origins. 
Most professionals agree that ethnicity estimates are accurate down to the continental level only. That is why you will get varying ethnicity percentages depending on which company you test with. 
Of course there are exceptions, and if you truly do get results that are way off the mark, then you may want to start taking a hard look at your close genetic matches.
One of the benefits of working with a professional genealogist is that we can help decipher your results in a way that is understandable and also meaningful. There is so much more to genetic genealogy than just your ethnicity percentages. If you are interested in exploring your genetic roots further, please contact us and we'll be glad to help you get the most out of your DNA test results.
 Amy Hackney Blackwell, The Myths, Legends and Lore of Ireland (Avon, Massachusetts: Simon & Schuster, 2011), 39; Google Books (https://books.google.com : accessed 4 June 2018).
 Bryan Sykes, Saxons, Vikings, and Celts: The Genetic Roots of Britain and Ireland (New York: W. W. Norton & Co., 2006), 280–1.
 Roberta Estes, "Ethnicity Testing and Results," DNAeXplained, 19 August 2015 (https://dna-explained.com/2015/08/19/ethnicity-testing-and-results/ : accessed 4 June 2018). See also Blaine Bettinger, "Problems with AncestryDNA's Genetic Ethnicity Prediction?" The Genetic Genealogist, 19 June 2012, (https://thegeneticgenealogist.com/2012/06/19/problems-with-ancestrydnas-genetic-ethnicity-prediction/ : accessed 4 June 2018).