There are a few things about DNA results that lead lots of people to mistaken expectations.
There's no guarantee that any particular ancestor's DNA will have reached you. Look back 10 generations, for example - the 17th century or so. In that generation, your family tree has 1,024 spots to fill. Those 1,024 people had 47,104 chromosomes. You've got 46 of them. 99.9% of that DNA didn't reach you. By some reckonings, I'm "3/8 Irish" because 3 of my 8 great-grandparents were born there, but there's no guarantee that 3/8 of my DNA came from them. The family tree fraction isn't the same as the DNA fraction.
I've got 12-14 people in my tree from 10 generations ago for whom I've got any halfway decent data. In other words, not only do I have less than 1% of that generation's DNA, I know who less than 1% of them are.
Another key point is the time frame of the DNA test. AncestryDNA says it looks back centuries or a thousand years or more. That could be 20-30 generations ago. I have very, very little info about my family tree back then. The German and Irish ancestors I know about, for example, were born about 200 years ago, but I know nothing about their ancestors from 500+ years earlier.
What it comes down to is that "surprises" in the AncestryDNA results are no surprise. The percentages haven't given me anything I can use for family research. The cousin-matching, however, has introduced me to a few cousins who are willing and able to swap info.