Tanya Chapter 2 part 3. So many varying levels of souls, all from one Source. Why the differences?

All Jewish souls originate in the wisdom of Hashem, yet we observe a multitude of differences among them. Let's revisit the analogy of the seminal drop originating in the father's mind. Just as all parts of the body, from the brain to the toenails, stem from the same essence, so do Jewish souls originate from one Source. However, as the drop undergoes a nine-month gestation period in the mother's womb, it progressively descends from its source, eventually giving rise to even toenails. Similarly, Jewish souls, having traversed the downward gradation of worlds, acquire lower levels of consciousness. The secret lies in our oneness, and by connecting with Torah sages who represent the "brain," we can come to know and manifest our own essence, no matter how low we have descended.