It's a commonplace experience among people who participate in surname group studies through a shared interest in genealogy. Test error is probably not the issue; missing paper trail is the issue 99% of the time. People who lost a spouse and other parent to their child or children didn't wait around, typically they'd remarry as quick as they could find an acceptable candidate. The adoptive parent becomes the most recent and therefore only parent in the record, and in a lot of old-time families none of that information got passed down.
There's also the "act like a fool factor", which no one wants to admit their ancestors did. In my experience, genealogists are really nice people who nevertheless like everything exact, cut and dried, and are absolutely sure that *only they* have the correct information.
In one of my surname groups, the test participants (1000s by now) are split into four groups; while all of them are, on paper, from common ancestors circa 400 years back, each of the first three groups are indeed of common descent, but don't match the other two large groups. The fourth group doesn't match anyone else in the study, except perhaps some first cousins.
I hadn't found any real black sheep until I found a 3rd cousin once removed in Bellevue, Washington, James Schumacher, who killed his wife with a hatchet in 2012.
I guess that's not an ancestor but it was through researching an ancestor that I finally tracked him down because I was looking for living cousins from lines they didn't know the others existed.
My little sis did a family line search some years back.
Found our family descends (direct patri line) from John McKay, brother of Alexander McKay, who came to the US in 1750.
That’s as far back as she went and as far back as anyone in the family is interested.
We’re American. No interest in who was who or whatever in them foreign devil lands.
See Comment #212 for more details. The most interesting thing I have is a German language genealogy dating back to the 1700s. This is from my maternal grandmother’s side. Some day I hope to get it translated. I am told that some ancestors were robber barrons. I know that my great grandfather lost the 1000 acres they owned in East Prussia (currently Kaliningrad) during the Franco Prussian War, 1870s. They had potato contracts with the army. There was a blight and they sold land to buy potatoes to meet the contract. They finished with 20 acres and the buildings. My great grandfather dropped von (sir) from his last name and went to work as a court secretary. His daughter married my grandfather who was a marine engineer. He didn’t like the way Bismark’s navy was headed and brought his wife to America. There is probably some Asiatic tartar way back there. I read that 1/20th of humans have DNA from Gengis kahn (sp?), who conquered as far west as East Prussia. Even though my husband had light blue (highly recessive color) and I have hazel eyes (moderately recessive), both sons have very dark brown eyes, although one has light brown hair. So I suspect it is a fairly strongly recessive Asian dark brown eye color, weaker than my main color but stronger than my husbands pale blue. The idea that we most might have blood of conquerors in our veins does not surprise me. Remember the law of Primogeniture? The custom that the lord had first rights to the new bride of a serf.