Well there was this Cracked.com article that 23andme quickly disavowed ... but ...
But when Inside Edition had a set of triplets send their spit in to Ancestry.com and 23andMe, they got wildly different results from both services. Neither gave each triplet the same ancestry results -- which, considering they all came from the same womb, is pretty weird.
So, who knows?
Some of the ethnicity estimates might vary because they are based on statistical comparison of markers in a person’s DNA to the prevalence of those markers in ethnic groups. While the distribution of markers is consistent for the group, an individual taken from the group might not have the same distribution of markers. They could have all of them or none.
However, other aspects of the DNA analysis are spot on. Another company has correctly identified all of my cousins and uncle who have also submitted DNA samples. And they tell me about new relatives all the time (I have thousands).