The lab that analyzed the dna samples was later found to be contaminated. Besides the dna in semen is only half of ones dna and to make it even more difficult the dna in semen cells is different from each other even from one person. In addition to this, the whole genome is usually never sequences, just parts of it. And when you sequence let's say a thousand base pairs, there will always be a change that more than one person (not identical twins) have the exact same sequence in that same part of dna that was analysed. Therefore as a result you can with certainty just outrule people, but you cannot say for sure that a person did it, if the dna matches. The longer the part sequenced or the more parts sequenced in ones genome, the more likely it becomes a match is more perfect match.