Having DNA from regions where none of your known recent ancestors were from really isn't uncommon at all, especially with respect to neighboring regions. The British Isles were invaded, conquered, and settled by Germanic Anglo-Saxons, Vikings, Danes, and later by the Normans (Scandinavian/French in origin). This explains the large overlap between Scandinavia and the British Isles in DNA inheritance. I think you might say that Scandinavian DNA is a shorthand for having "Germanic" ancestry of some type (be it Viking, Norman, Anglo-Saxon, Frankish, Gothic, Lombard, etc.). I have a somewhat similar situation in that I have 17% Scandinavian DNA but no known or recent Scandinavian ancestry, and I'm attributing most of this to my partial Anglo-Irish ancestry. Likewise, I also have 10% Caucasus and 4% Middle East DNA in my background too, without having any known or recent ancestry from these regions. This DNA I attribute to my significant Sicilian/Italian heritage and it could be the result of Phoenician colonization, Arab settlement in the Middle Ages (Moors), or slaves from the Eastern Mediterranean/Middle East brought back to Italy by the Romans. DNA testing like this really allows us to see how people from neighboring regions overlap with each other and how our ancestors really moved around a lot more than we realize.

My DNA is a total mishmash of various Mediterranean/European groups (Italy/Greece, Scandinavia, Caucasus, Iberian Peninsula, Britain, Middle East, and others), which I wasn't at all surprised to discover considering the histories of the lands where my ancestors came from as well as my physical appearance (I've been guessed as everything from "Black Irish" to Palestinian/Lebanese as far as ethnic origins go, with others still readily identifying me as at least partially of Italian/Greek or generic Southern European origin).