The assumptions behind the testing were biased by the preconceived ideas of the time. Only a small part of the mtDNA is tested for species identification. When the markers in that area match, you assume match. In the past, because of the cost, and because of the assumption that bigfoot was just an ape, an upright gorilla as some still assert, when the initial pass shows human, not ape, at the gene locus (locii?) that are used to separate human from known ape, then the testers assumed human contamination and stopped testing. It is likely if those were real bigfoot DNA, if they had continued testing, they would have found differences.
I don't think it is right to conclude that those samples WERE bigfoot DNA.
What it is proper to conclude is that the testing was terminated before going far enough to make any determination other than whatever provided the DNA, it was very very humanlike if not actually human.
There is another possibility if this is taken to extremes. It's not as simple as specific genes being present or not. Some genes can be present but "turned off" by the actions of other proteins, RNA for instance, which determines whether the trait the gene is for actually manifests itself. It could be possible ... I'm not claiming this is true, but possible ... that the DNA could be essentially identical but different pieces active or inactive because of some slight difference on a completely different section of the DNA producing a protein that toggles it one way or the other.
This stuff is NOT simple and people who are trying to make it so are doing the rest a disservice.
For all I know, those seemingly contaminated samples were just that. I'm throwing out possibilities, I'm not doing a Giorgio Tsoukalous imitation jumping from "might be" to the assumption of "must be" or "is". What I believe we know now and all should agree on is that the past testing didn't go far enough, we assumed it was adequate to prove human contamination or not, and we don't know that for sure anymore. I wish we had some of those old samples to retest with today's techniques and at today's costs.
MIB