No one who posts on this blog knows precisely how the JSS is calculated.
There is at least one major hidden factor - private feedback from clients. It is likely never substantially higher than public feedback, but there is general agreement it can be substantially lower than public feedback (or even highly negative when the public feedback is highly positive).
This subject has been covered ad nauseum on this board. You can easily find many threads with "JSS" and "Job Success Score" and "feedbback" if you search this board. There isn't anything new to say on the subject.