Assalamualaikum,
The most popular view is that the division into 30 Juz was done by Hajjaj bin Yusuf, and so was the introduction of the A'raab, and the division into Ruku's. The juz were numbered 30 so that the recitation of the complete Quran could be completed in 30 days, however the contents of the Quran were not considered in this. For example, Juz 13 contains 1 Ayat of Surah Hijr while Juz 14 contains the rest. Juz 21 ends with:
O wives of the Prophet! If any of you were guilty of evident unseemly conduct, the Punishment would be doubled to her, and that is easy for Allah.
Juz 22 starts with:
But any of you that is devout in the service of Allah and His Messenger, and works righteousness,- to her shall We grant her reward twice: and We have prepared for her a generous Sustenance.
As can be seen, the Juz are not organized according to the meaning of the words, just the number of pages has been considered.
On the other hand, the Ruku's are, more or less, very accurate, in that the beginning of each new Ruku normally signals the beginning of a new 'topic'.
Hope this was helpful.
Wassalam