To simplify, think of it this way, if you hatched eggs of a breed that was supposed to have a single comb but one or two of them hatched with a pea comb, would you say that it must be pure but it just somehow spontaneously has a pea comb? Or would you suspect it was a cross? How about if you hatched a bird that was supposed to be a marans, but it laid blue eggs? Or a breed that was supposed to be solid black, but you hatched a chick that was barred instead? Would you think all those were purebreds, or would you suspect an oops breeding happened and they were crosses? Those are all the same situation as this.