Allama Mohammad Baqir al-Majlisi is a great scholar in the Hadeeth sciences and he compiled the well known collection of Hadeeth in his famous book Biharul Anwaar. He has written others books as well. His grading is according to his own research and Ejtihaad. Other scholars have their grading which can be like or different from his grading as all the Mujahid scholars are free in their research in the narrators and narrations.
Al-Kaafi book has the most authentic collection of narrations of Hadeeth but we don't say that every narration in al-Kaafi is equally authentic. The expert scholars have their right to study and verify every narration.