Thank you for your question. Different authors had different purposes and differing criteria when compiling their books. Sometimes, it is not the author's criteria, but the version of the book that has reached us that may be problematic, as Shii hadith history is a history of textual transmission.
Bihar al-anwar is a book that was written in the 17th Century which tried to preserve as much of the Shii tradition as possible. The author was therefore not concerned with sifting reports and left that to later hadith scholars, as hadith scholars also use different criteria when analyzing the reports in various books.
Usul al-Kafi on the other hand was an attempt to produce a book of reliable reports, but at the same time, not everybody agrees with what the author of al-Kafi (al-Kulayni) considered reliable. In short, hadith scholarship is a complex field where there are many aspects to weigh up and that is why it takes expertise. The natural outcome of that is that not everyone will agree with a specific scholar's assessment if they are trying to produce a reliable work.
May you always be successful