I found this:
" What is a partial match at NDIS?
A: Occasionally a partial match between a forensic profile and an offender profile is observed during a routine NDIS database search. The FBI defines a partial match as a moderate stringency candidate match between two single source profiles having at each locus all of the alleles of one sample represented in the other sample. (See below illustration). A “partial match” is not an exact match of the two profiles. A forensic scientist, when evaluating whether a candidate match is a viable match and should be processed through to confirmation, discovers that the candidate offender profile is, in fact, excluded as the possible source of the profile obtained from crime scene evidence but that, because of a similarity in alleles between the forensic unknown and the candidate offender profile, believes that a close biological relative of the offender may be the source of the forensic unknown."
http://www.fbi.gov/about-us/lab/biometric-analysis/codis/codis-and-ndis-fact-sheet
Sounds like this is what happened