Genomic imprinting is the epigenetic modification, by which certain genes are expressed depending on their parent specific origin. If certain genes inherited from father (paternal genes) are imprinted, they are silenced, and only genes inherited from the mother (maternal genes) are expressed.
In a diploid organism, two alleles are needed to express the recessive phenotype. Deletion of any one of the two alleles will result in the dominant phenotype. Same is true in the above given case. When an imprinted allele is deleted by a mutation, the remaining one allele is enough to express the phenotype.