How does bcftools isec identify the same sites

[ym-chen]

I used bcftools isec to find common sites in multiple vcf files. But I found a confused site. The records from vcfs are:

vcf1 chr3 183987815 . G A . PASS AS_FilterStatus=SITE;AS_SB_TABLE=152,144|24,25;DP=351;ECNT=1;GERMQ=93;MBQ=38,31;MFRL=288,304;MMQ=60,60;MPOS=43;NALOD=2.37;NLOD=68.24;POPAF=6;TLOD=129.08 GT:AD:AF:DP:F1R2:F2R1:FAD:SB 0/0:236,1:0.00432:237:114,0:98,0:234,1:118,118,0,1 0/1:60,48:0.448:108:28,23:26,21:58,47:34,26,24,24 vcf2 chr3 183987815 . G T . PASS AS_FilterStatus=SITE;AS_SB_TABLE=210,249|22,22;DP=507;ECNT=1;GERMQ=93;MBQ=38,33;MFRL=292,282;MMQ=60,60;MPOS=39;NALOD=2.31;NLOD=60.38;POPAF=6;TLOD=100.18 GT:AD:AF:DP:F1R2:F2R1:FAD:SB 0/0:203,0:0.004889:203:109,0:88,0:202,0:85,118,0,0 0/1:256,44:0.145:300:119,23:127,21:254,44:125,131,22,22

Even though the alts of these two sites are not the same, isec still considers these to be a common site. It's not what I expected. I wonder why isec thinks these two sites are the same.