At the continental level for DNA ethnicity results, if you have a sizeable amount of a given continent, it’s probably valid. If it’s trace or small amounts, it may be noise. Below the continental level, I encourage people to ignore them. There are several reasons – first, the science is nowhere near accurate enough at this point to be reliable. Different companies will give you different results. The companies use different reference populations and algorithms to generate the results. Lastly, there are very few places on the planet where a population has stayed mostly ethnically pure and not mixed with other ethnic groups.
Using Europe as an example, most European countries have been invaded, peacefully or not so peacefully, for millennia. Here’s a good YouTube video – History of Europe (3000 BC – 2013 AD) – YouTube: https://www.youtube.com/watch?v=l53bmKYXliA. It’s around 5:32 minutes long. As this and other videos show, European countries are a mix of various ethnic groups. I’m about 80% British/English per family history, but the United Kingdom is full of different ethnic groups – French, English, German, Viking, Spanish, Roman, etc. End result is my English percentage is intermixed with descendants from the other ethnic groups.
For Jewish, while most Jews married within their own group, many did not. I have a female friend who fell in love with a Jewish guy. Before his parents would let her marry him, she had to convert to Judaism as their Jewish group requires the mother to be Jewish. She converted and they have four children. However, as she wasn’t born Jewish, the children will be maybe 50% Jewish in terms of DNA, depending on the percentage of non-Jewish DNA in her husband and his family’s DNA. In a few generations, the story may be lost that she was a converted Jew and the descendants may be surprised as the amount of non-Jewish they have in their DNA.