From the NY Times article: Chimerism--having cells from another person in your body.
One route to this odd state, called chimerism, is the vanishing twin. Dr. Helain Landy of Georgetown University, who has no involvement in the Hamilton case, has found that 20 to 30 percent of pregnancies that start out as twins end up as single babies, with one twin being absorbed by the mother during the first trimester.
Others researchers have found that in some cases, before the twin is absorbed, some of its cells enter the body of the other fetus and remain there for life. The cells can include bone marrow stem cells, the progenitors of blood cells.
Another route to chimerism is through the cells that routinely pass from a mother to fetus and remain there for life.
Perhaps instead we should think about reinterpreting the meaning of the DNA analysis. Does the "chimerism rate" represent an error rate in the tests?
So many "breakthroughs" have been anounced based on DNA analysis that I'm starting to become quite skeptical. Every other month or so we hear that animal X is really related to animal Y instead of the animal Z that it looks just like.