"And people have made clear to me that the test given is the same test wherever you take it."
Well... Not so much. Without the strangling detail I can tend to go into :-) autosomal DNA tests (yDNA and myDNA tests don't necessarily employ the same process, though some do) use some form of microarray chip, most of them manufactured by a company named Illumina. Over 90% of the world's genetic sequencing data is done by Illumina chips (there's a plot for a science fiction novel in there somewhere, but I won't touch it).
Some testing companies use customized Illumina chips, and some use the Illumina OmniExpress chip. Even if they use the same chip, it doesn't mean they use the results the same way...or even use the same data the chip provides. For example, when you look at the number of autosomal Single Nucleotide Polymorphisms (SNPs) tested, you see differences:
- 23andMe: 577,382 SNPs tested
- Family Tree DNA: about 690,000 SNPs tested
- AncestryDNA: 637,639 SNPs tested
- Genographic Project Geno 2.0: 700,000 SNPs tested
- MyHeritage: 702,442 SNPs tested
These numbers, though, pale when you consider that the human genome has about 3 billion bases to evaluate. To put that into perspective, 700,000 tested SNPs would represent about 0.0233% of the whole genome.
How the different companies evaluate results, and how/what they display, is a whole 'nother story.