How does GATK caculate the HaplotypeScore in Unified Genotyper

Question

[naruto1nrt]

I have a snp with HaplotypeScore=14 which must be filtered by hard filter(HaplotypeScore>13). But I am sure about this snp is true.

So do anybody know how does GATK caculate the HaplotypeScore in Unified Genotyper?

Thanks