Why do students shubah and hafs differ with assim in qirat

Question
Why do students shubah and hafs differ with assim in qirat
Answer
The early scholars of the Qur'an and the different qira'aat learned many different turuq and passed down some of the authentic turuq to various students and other authentic turuq or variations to others.  The variations between Shu'bah and Hafs are minimal compared to other qira'aat and the two famous raawee for each.  Imam 'Aasim chose to relate some special words from one tareeq to Shu'bah and an another to Hafs on a few occasions.  There are not many, but in other qira'aat there are many differences.  All of this goes back to the vast knowledge of the scholars.  Each way passed down is well documented and validated by scholars of the Qur'an and the qira'aat.  It shows the miracle of the Qur'an and its transmission and how specific and detailed this transmission has remained over the centuries.