But, if you have DNA from 2 different people as someone up thread had mentioned, 2 brothers, or a man and his cousin, the DNA would be convoluted and need further analysis to separate. Thus, not leading to an exact match. I personally think this is highly probable in this case.
Additionally, I wonder if one of the persons could be related even distantly to NG, and her DNA and the 2nd perp's might be convoluted. Not to say a family member is involved as some here have speculated, but just throwing out scenarios that might be complicating the separation of the DNA.