If it shows in your DNA I would believe it. But if you're going by family trees, like other people have said, they are just what other people are adding and some people don't take the time to use facts. They just copy and paste from other people's trees. If ancestry is showing that you are related to someone then it's through DNA. It doesn't always show how because some people don't have trees or not a lot of people in their trees so you can't match the names. But as someone else also stated, you could always contact the person and start a discussion with them. But never take anything as fact from someone's family tree unless there is proof/documentation to show it's a fact.