Answer: ZA is made of six parts because Keter, Hochma, and Bina precede it and it is born from Bina. When ZA is born, it lacks the first three Sefirot: Keter, Hochma, and Bina.
Therefore, it contains only six parts: Hesed, Gevura, Tifferet, Netzach, Hod, and Yesod. Then comes Malchut. This means that there are the first three Sefirot in the Partzuf, six Sefirot, and one more, which all together makes ten Sefirot.
From the Kabbalah Lesson in Russian 3/5/17