There is a difference between ראש (head) and קדקד (top of the head): The noun קדקד refers to the upper side of the head, and the verb that is derived from it ויקדו (nod) is the bending of the top of the head downward, as in the verb כריעה (kneel) which refers to the bending of the כרעים (knees). But the noun ראש (head) includes all of the head.

And we find that כי ימרט ראשו (And if a man's hair be fallen off his head, he is bald; yet is he clean)⁷¹³ refers to the back of his head alone – and there is a homiletical interpretation.

footnotes: ⁷¹³ Leviticus 13:40.