It's amazing that when the machine was pushed past the parameters they needed, it happened to give results that matched exactly with Meredith's DNA, don't you think? The expert who did the DNA test on this knife is also the person who ID's Guede's DNA at the scene. If she's incompetent and lied about Meredith, why should we trust her on Guede?
That's simple -- they can bring in a whole gaggle of experts to test Guede's DNA again and again and again because they have so much of it from the crime scene.
But they have no more knife material -- so no more tests can be done.
They should have used the same guidelines for both materials and let it stand -- but they didn't.
It's called a double standard. It shows intent on their part to bend what ever rules they had to to get the results they wanted.