11th March 2013, 08:44 PM
I'd imagine it's because they can't prove that there isn't someone else with the same DNA profile (albeit how unlikely), whereas they can definitively prove that two sufficiently differing results can't have come from the same person - at a guess. For R3 all they could really show decisively was that it wasn't him (assuming that his mother was really his mother), the result they've got has instead shown merely that it could be him