Look, its very simple.
1. Orochimaru is a subset of everyone.
2. Everyone is Zabuza in disguise.
3. Zabuza is secretly Orochimaru.
This initiates a recursive Orochimaru/Zabuza cycle. Every 3rd iteration splits off both a Spin and Antispin Itachi, while every seventh iteration produces an Up-Sage of the Sixth Path. Cases where both an Itachi-pair and a Sage would be generated instead produce a Spin Itachi and a Down-Sage.
This is what we call Chakra.
Now, this does mean that there is a slow accumulation of Spin-Itachis and Up-Sages, which is actually where we get Orochimarus and Zabuzas in the first place.