DNA testing is not an easy answer to uncovering your family history, but it is a helpful tool. And like any other tool, you have to know how to use it. But DNA is tricky and takes some work to understand. There are many variables that can affect your results and can cause confusion. That said, I'm no expert. If anyone wants to add or correct any information, feel free to comment.
First, understand that in this post I'm referring to autosomal DNA test results, which predict relationships between you and other testers. Results typically find relationships back to about 5th or 6th cousins (a shared 4x or 5x great-grandparent). So, how difficult could this be?
Have you heard of Pedigree Collapse? This is where you descend multiple times from an ancestor. For example, Moses Cash and Nancy Hudson are twice over my 5x-Great Grandparents. Both of their daughters, Sarah and Elizabeth, are my 4x great grandparents - one married an Alexander and one married a Craft. Eventually, their great-grandchildren would marry. So what does this do to DNA results? It makes you appear to be more closely related than you are.
In this example, I was given a suggestion of 4th cousin to a match who was a descendant of Moses Cash's grandparents, Stephen and Jemina Cash. In reality we were 7th cousins once removed. To compound matters, the lady I matched with was also twice over related to Stephen and Jemina Cash and I have double Craft ancestors. What looked like a close family connection, was actually a rather distant one, made confusing thanks to our double doses of Cash DNA.
Here's something strange you might see in your family tree: you're more closely related to your match than your mother or father is. For example: I have DNA a match with another genealogy blogger, Tonia (check out Tonia's Roots) We're suggested to be 4.3 generations removed and share 35.5 cm of DNA. My father is suggested to be 4.4 generations removed and shares only 30.5 cm of DNA with Tonia. How is this possible? He should be more closely related to all of my DNA matches than I am.
It turns out that the extra 5 cm of DNA comes from my mom. She's recommended to be 7.7 generations removed from Tonia and shares only 5.4 cm of DNA. This is a much more distant relationship, but a relationship nonetheless. Given the fact that my parents don't share an ancestor within a genealogical time span, this is not an instance of Pedigree Collapse. Instead, it means that Tonia and I have two different common ancestors, from two different sides of my family. And this is not the only example of this I've come across - and boy does it confuse things! Again, this is an example of being predicted to be more closely related than you are.
These are just two examples of how your results might indicate one thing, but when you look at your paper trail, you realize that there's actually something else going on. When looking at your autosomal DNA test results, look carefully and closely; you never know what you'll find.