The way I heard it ("rumor" and I have lost the web link to it also) is that one of the profiles that they are using as a standard for what is "Scandinavian" has since been judged by others in the genetics field as "non-Scandinavian". As in the person(s) thought they were of pure Scandinavian descent and it turns out they were not. Since AncestryDNA is still using that profile as one of their standards they are telling people they are Scandinavian when they might not be.
I'm a perfect example of this. My test came back as 90% Scandinavian, 7% Southern European, 3% unknown.
My mother's is 85% British, 10% Scandinavian, 5% unknown.
Admixture is definitely off.