we're all connected in some way or other!
When I first took the test nearly two years ago, Ancestry had outlined about 25 or 30 regions in the world, uniquely identified by DNA commonality. These regions were divided not by political boundaries, natural features or continents, but according to the uniformity and uniqueness of DNA found in residents of those regions. DNA samples from living people who have a long ethnic history in a region are the standard comparison, and to make it simple here, as your DNA closely resembles the deep-rooted samples from one of those regions, Ancestry can quantify what percentage of your ancestry comes from that area.
Currently Ancestry's regions have expanded to now number 350, thus their customer's DNA can be honed to a small area. Their sample populations have increased by a factor of at least three, further refining the matching process.
Here is my updated 2018 Ancestry DNA map showing my ethnicity. Look at Ireland, where 50% of my DNA originates. Notice the small circle drawn around the very most southwest part of the country. This is exactly where almost every one of my Irish ancestors come from. Now look at the leaf-green oval drawn around the point where Poland-Ukraine-Slovakia-Romania come together. 45% of my DNA originates from there. Both of these regions I long-ago identified as domiciles of my ancestors, and now DNA testing has corroborated that. Amazing!
NOTE: my only DNA surprise involves 5% of my DNA: 4% of that is Baltic, and 1% is from the Balkans.