Germline mutation with t_alt_count 0 identified with GATK

I used GATK tools haplotypecaller and GenotypeGVCFs, and identified germline mutations.

One of the mutations look like

after GATK GenotypeGVCFs"

V1 V2 V3 V4 V5 V6 V7
1 16257 . G C 44.77 .

                                                                                                               V8

AC=2;AF=0.250;AN=8;BaseQRankSum=0.00;ClippingRankSum=0.00;DP=13;ExcessHet=3.6798;FS=0.000;MLEAC=2;MLEAF=0.250;MQ=30.51;MQRankSum=-9.670e-01;QD=6.40;ReadPosRankSum=0.967;SOR=0.368

          V9                      V10                          V11                              V12

GT:AD:DP:GQ:PL 0/1:1,2:3:19:40,0,19 0/0:4,0:4:12:0,12,99 ./.:0,0:0:.:0,0,0
V13 V14 V15
0/1:2,2:4:37:37,0,37 0/0:2,0:2:6:0,6,49 ./.:0,0:0:.:0,0,0

After oncotator annotation:
t_ref_count t_alt_count depth_across_samples genotype
0 0 13 ./.
genotype_quality mapping_quality qual read_depth total_allele_number
NA 30.51 44.77 0 8
maximum_likelihood_expectation_allelic_count
2
maximum_likelihood_expectation_allelic_frequency phred_scaled_likelihoods
0.25 0,0,0
variant_quality_by_depth z_score_base_qualities
6.4 0
z_score_read_mapping_qualities z_score_read_pos_bias
-0.967 0.967

How come this is identified as mutation if both t_alt_count is 0?