It can be thought that this is because Chinese Tendai sect (Tiantai-zong) incorporated the Nehan sect (Niepan-zong) which was based on "Daihatsu Nehan-gyo" (Daban Niepan-jing, the Nirvana Sutra), one of Mahayana Sutra of the middle period, in which the term 'bussho' (仏性foxing) appeared for the first time.
これは仏性が初めて説かれる中期大乗経典の大般涅槃経を依経とする涅槃宗を中国天台宗が吸収したことによるものと考えられる。