why and how multi allele gets reported during variant calling in vcf?
In your example the 4th column contains the reference allel on the given position. It is not said, that your sample has this. The 5th column contains all alternative alleles found on the given position.

Which alleles are present in your sample is given in the last column by 1/2. This means you have one allel with the first value in the 5th column and one allele with the second value given in the 5th column. You see, that your sample has exactly alleles, but none of them is the reference. This would be indicated by a 0.