Why is the short forms for William "Bill", Robert "Bob" and Richard "D|ck"?

I understand William being shortened to Will, Kirkland being shortened to Kirk, Maximus being shortened to Max and Billy being shortened to Bill. But how can Bill be the short form for William if there is no "B" in William, how can Bob be the short form for Robert if there's no "B" at the start of the name, and how can Richard be shortened to D|ck if there is no "D" at the start of Richard? I've always been confused by this.