Relationship projections in DNA test results are estimations based upon shared DNA.
The relationship is an estimate.
The relationship stated is based on the amount of shared DNA (measured in centimorgans). The Shared cM Project has estimates of relationships based on shared centimorgans. These are estimates. These probable relationships on the Shared cM Project are based on submissions of known relationships and the known shared centimorgans between those individuals. Each individual occurrence is anecdotal–it’s the accumulation of data submitted that is used to create the averages and ranges. It’s statistical data based upon submissions. You get half your autosomal DNA from each parent. Your parents got half of their autosomal DNA from their parent. Your grandparents got half their autosomal DNA from each of their parents.
But that does not mean that you got exactly one-eighth of your DNA from each of your great-grandparents.
When your father’s body is creating the genetic material that will be passed on to you (the autosomal DNA anyway), there is no one standing there saying: “let’s make certain that we pass exactly half of grandpa’s DNA to this child and exactly half of Grandma’s DNA to this child.” There is similarly no one overseeing the mother’s passing of autosomal DNA either. The body does not look at the autosomal DNA and make certain it is passed to the next child based on ancestral proportions.
While the metaphor here does not accurately describe the biology, it does explain the process. Let’s say your Dad inherited fifty one dollar bills from his father and fifty dollar bills from his mother. He does not spend the money, shuffles the bills around, and later decides to give you one half of that money.
Those bills you get (half of what your father had) may not be exactly twenty-five of his father’s bills and twenty-five of his mother’s bills. It could be twenty-seven from one and twenty-three from another. But you still got fifty. And…without going into detail the amount you got from each is probably close to have. Probably close and not exactly. The problem is that probably close and not exactly gets magnified as the generations go further back.
Because the autosomal DNA from your grandparents and earlier generations of your family is not equally split in your genetic makeup, predicting biological relationships beyond a certain point is an estimation. That’s why the Shared cM Project was started.
And that’s why my third cousin (we’re both great-great-grandsons of the same couple), shares a smaller amount of DNA than expected. AncestryDNA predicts that our relationship is between 5th and 8th cousins. According the current version of the Shared cM Project, third cousins can expect, on average to share 74 centiMorgans of DNA. Expect. On average.
99% of third cousins share between 0 and 217 centiMorgans of DNA according to the Shared cM Project.
For what it is worth, the predictions of my relationships with individuals on AncestryDNA have been relatively accurate with first and second cousins who have tested. Most of third cousins have been relatively accurate. Beyond that it is best to remember that the predictions are just that: predictions.
Your autosomal DNA test results should be used as one more genealogical tool. They are not the only tool and they should not be used in isolation.