1, Yongzheng had great attainments in eight-character numerology, and Qianlong's blessing should be the best among all emperors in China. Yongzheng personally calculated that this son's life is super good. Therefore, please keep it dry and store it for a long time. This is the main reason. This work habit can also be inferred from Yongzheng's behavior.

2. Because Ganlong's mother's surname is Niu Kelu, Ganlong is the only son born to a Manchu woman among all the sons of Yongzheng. From a national perspective, it is also a priority to establish a dry dragon. (The information of Gan Long's mother is subject to the official history for the time being)

Another reason may be that Kangxi likes Li Hong. As for why he kept a secret deposit, it was to avoid a fight between brothers.