Contrary to popular belief, there is no such thing as Welsh, Irish, Scottish or indeed Viking DNA. Nor, if you read the small print, do companies such as Ancestry claim there is.
What Ancestry actually does is match your DNA sample against other samples from their database which are considered to represent a particular ethnicity because they and their ancestors have lived within a particular part of the world for several generations.
So having "Scottish" ethnicity does NOT mean you have "Scottish" DNA it simply means you have some DNA which is common in people living in Scotland today, or whose families lived in Scotland for many years.
My own ethnicity for example is registered as 96% Scottish, which makes sense since virtually all of my Ancestors do indeed originate here, but given that my maternal grandparents lived in the COwal Peninsula for generations, it seems exceptionally unlikely that none of them married into families of Norse descent somewhere along the way. So why does almost all of my DNA inheritance come up as Scottish?
Simple. The Norse settled so widely down the coasts of Scotland that practically everyone living in Scotland today has some Norse descent.
As I understand it, the situation goes something like this. Ancestry slices up your DNA into fragments and matches those fragments up with it's databases. A specific fragment will generally show up in varying amounts in several different groups, but Ancestry assigns it to the database which has the highest percentage of the population having that fragment. So if the vagaries of war, famine, population migration and dominant or recessive genes means that 1000 years after one of my ancestors inherited a specific DNA fragment from a Norse settler, that fragment is now more common in those from Scotland than those from Norway, Ancestry assigns that fragment as being "Scottish."
Contrary to all you may see, that's all it does. Ancestry (and every other similar company as far as I am aware) does not say I am "Scottish." All it says is that I share more DNA with people in its Scottish Database than any other.
Having said all that, I agree with the several posts stating that DNA is much more valuable in finding matches to your tree than in ethnicity for those researching their family history. The ethnicity estimates can be a useful pointer, but little more.