It was because in Japanese ritsuryo system, each sho (Ministry), shiki (agency), tsukasa (office) and ryo (bureau) had a certain independence, in addition, it is said that it arose since government officials (such as kenmotsu and a judge) which had existed and been independent before the ritsuryo system were forced to be added into jurisdiction of a specific sho with keeping the traditional independence, and as a result, many honkan gathered especially in the Nakatsukasasho which took charge of general affairs within the Imperial Court.
これは日本の律令制が省・職・司・寮それぞれに一定の独立性を持たせたこと、加えて律令制以前に存在していた独立していた官司（監物・判事など）を従来の独立性を維持させたまま、強引に特定の省の管轄に加えたために生じたとされ、特に朝廷内部の事務一般を管轄した中務省には多くの品官が集中することになった。