In the end when you love your dog, things like this don't and shouldn't matter to you. We only want to know things about genetics because we love the breed, and as
@Panama says curious minds want to know more about genes & genetics - coat color and length is all tied to certain genes - and since this isn't a variety listed in Dobermans (that I know of) we just wonder how the gene got there.
Love your dog - would love to see more photos and tell us more about him & his personality! Have you had him since a puppy or did you just get him recently?