This question is intended only for those familiar with some basic Kabbalistic concepts.
As we know (from יהו"ה), that ו (Vav, number 6) stands for Z"A (masculine) and ה (He, number 5) stands for Malchus (feminine).
However, the Written Torah - that is Z"A - has 5 books, where the Oral Torah - that's Malchus - has 6 Sdarim.
How come?
Perhaps it is a play on a midrash Devarim Rabba 3:12 that prior to being written on a parchment the Torah existed as black fire on white fire:
Reish Lakish said: When he wrote the Torah, Moses received the radiance of his face. How so? Reish Lakish said: The Torah that was given to Moses, its parchment was white fire, it was written with black fire, sealed with fire, and bound with fire. As he was writing, he wiped the quill in his hair, and it was from there that he received the radiance of his face.
I.e. the written law "black fire" is enveloped by the oral law i.e. "white fire". If you place 5 black fire columns on white fire, you will get 4 white fire spaces between them (maybe that's the origin of Arba'ah Turim) and two additional spaces on the sides. I.e. our 6 orders of mishnah are like the shin with 4 teeth on the tefilin shel rosh which envelops regular shin from the name Sha-ai.
Alternatively, it could be a way of completion of the name yud-hey-waw-hey as in the passuk Shemos 17:16:
וַיֹּ֗אמֶר כִּֽי־יָד֙ עַל־כֵּ֣ס יָ֔הּ מִלְחָמָ֥ה לַיהוָ֖ה בַּֽעֲמָלֵ֑ק מִדֹּ֖ר דֹּֽר׃
He said, “It means, ‘Hand upon the throne of יה, for Hashem will be at war with Amalek from generation to generation.
