There are a number of validity and reliability challenges that have been made against the ethnicity percentages. Here is another blog post, this one from Roberta Estes, in which she goes into great detail on this:
Specifically, she focuses on three issues:
"1. In determining majority ethnicity at the continent level, these tests are quite accurate, but then you can determine the same thing by looking in the mirror. I’m primarily of European heritage. I can see that easily and don’t need a DNA test for that information.
2. When comparing between continental ethnicity, meaning sorting African from European from Asian from Native American, these tests are relatively accurate, meaning there is sometimes a little bit of overlap, but not much. I’m between 4 and 5% Native American and African – which I can’t see in the mirror – but some of these tests can.
3. When dealing with intra-continent ethnicity – meaning Europe in particular, comparing one country or region to another, these tests are not reliable and in some cases, appear to be outright wrong."
So - as she and several other DNA genealogists have stated, the ethnicity percentages are reliable and valid at the continent level. If you have European ancestors, your ethnicity percentages will include European results. If you have African ancestors, your ethnicity percentages will include African results.
The ethnicity percentages are neither valid nor reliable beyond the continent level. The example that is frequently cited is the Ancestry DNA commercial in which a person exchanges lederhosen (Germany) for a kilt (Britain). While this might be appealing for a commercial, the test results cannot be taken as evidence.
In your case, I would watch putting too much faith into an ethnicity test result that shows Scandinavian ancestry while you were expecting German. These are both European, and beyond this, they are very close geographically. The current approach to ethnicity percentages simply does not offer consistent valid and reliable results on ethnic differences within a continent.