Six fingered offspring are formed from normal fingered parents . why?

Polydactyly in its most common form, is an autosomal dominant mutation. Let's represent the mutation as P and the normal state as +. Thus, if both parents are heterozygous (P+), both would have the mutation (and thus have six fingered). They have 3/4 chances of having a child with the extra digit, 1/4 normal.