These tests aren’t as accurate as their promoters would like customers to believe.
The technicalities are such that such tests have percentage accuracy variances, which I’m sure are in the fine print explaining each test.
Here’s one check of tests’ accuracy:
The tests are actually quite accurate. It is the interpretation that is the tricky part.
I was just poking around on Ancestry.com, and it showed me a name that it had determined is a very close relative, based on our DNA results. Yep, I think my uncle is a close relative. My cousin popped up there, too, and it placed her as actually being more distant than a first cousin--but that makes sense because she and I share a grandfather, but not a grandmother.
Another factor I look at is the ethnic profile determined by DNA. I compared my uncle, my cousin, and my ethnic background. Uncle has Bantu and Indian ancestry like I do, but my cousin (his niece) does not.
Some variations in DNA are due to the nature of the molecule. Even identical twins do not have identical DNA.
I think I probably understand the nature of the tests run by companies like Ancestry.com better than the average person. I did my Ph.D. studying DNA.