I don't think you're missing anything. I think Nifong is missing something, and that something is.....evidence, including admissable DNA evidence! ;>
So from Vn_survivor_67-68's post it would appear that a failure to match all 13 is generally a failure to extract some of the 13 from the "tissue" which makes the "tissue" compromised.
This makes more sense to me than successfully extracting all 13 from the "tissue" and then having some of them mismatch. It seems to me that if even there is one mismatch from a successful extraction of 13 then that would be the basis for an overall "no match".
David Evans
Matt Zash
Dan Flannery