There are several misled comments here.
The DNA tests don't confuse fraternal and identical twins. Therefore, forget whether you think these two women are identical. They are. Or are being purposely misleading.
The DNA outfits report around 600,000+ "SNPs" - pairs of molecules (one from each parent) that are of 4 types, labelled "A", "G", "C", and "T". The DNA companies choose their 600k+ SNPs from our 3.5 billion SNPs. The ones they choose vary in A, G, C, and T value between different humans. Almost all of our 3.5 billion SNPs are the same for all of us.
Each company chooses a different set of SNPs to report. 23andMe has reported at least two different sets of SNPs, depending on when you did their test. I've found Ancestry,com and 23andMe tests have from 100K to 300K of the same SNPs between any two test/versions (out of the 600k+ each test reports).
Two tests for the same person vary by only a handful of SNPs. That is, there is noise, but it's not very loud. Different companies' tests report different SNP values, too. Again, very quiet noise.
The serial killer thing in the article used these DNA kit results. Crime labs use something quite different.
"Ancestry" or "ethnicity" is calculated using fancy arithmetic. Different companies do, indeed, report different "ancestries". As would be expected. Issues, among others:
1) Different SNPs between companies and company-versions.
2) Ancestor? When? These guys generally are shooting for a few hundred years ago in Europe, not thousands. But outside Europe? Read on.
3) How much (if any) data backs up "ancestry"? You can't find what you don't know. These outfits have SNP combo examples from south-south-eastern Duchy of Euroland, but have bupkus for much of the world. Only recently have they started differentiating Siberia from Chile, for instance. And Siberia and Chile diverged over 10 thousand years ago!
4) Ancestry calculations depend on knowing which AGCT of the SNP pairs go to which parent. That's rather a trick to know when you don't have DNA data for either parent. The article doesn't say whether these women got their mother or father to do a test so such information could be nailed down.
The FDA doesn't care about "ancestry" reports. They "FDA'd" 23andMe a few years ago because of health reports. 23andMe has since jumped through the FDA hoops and now benefits from the usual lack-of-competition regulation causes. Don't hold your breath waiting for useful health information from these DNA tests.
I'm Scandinavian to two of the DNA outfits, Brit to one of them, and changed from Scand to Brit at a fourth outfit a year ago. Smaller percentages change often. These guys are busy tweaking their code and data.
23andMe, for one, allows you to choose how reliable you want their ancestry information to be. The article doesn't say anything about the settings these two women used. If you want 23andMe's most reliable report, don't expect a lot of specific ancestry information. "Northern Europe", folks. Or "African". Sort of at the level our eyes see without close examination.
Hope this helps.