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JAM is a postgraduate entrance, not a school one. It leads to M.Sc. and integrated research programmes at the IITs and IISc, and candidates sit a single subject paper, chosen at the time of applying. That one choice shapes everything afterwards: this is one subject taken deep, not three taken broadly the way the engineering and medical entrances are.
The syllabus is undergraduate, which is where the difficulty usually starts. B.Sc. courses differ a great deal between colleges and universities, and a topic given three lectures in one place is a full paper in another. A candidate who does badly on a practice set often cannot tell whether the chapter is weak or was never really taught.
So subject depth in the tutor counts for more here than exam tactics. What most JAM aspirants are after is someone who has studied the same subject to postgraduate level, can teach at that level, and can help decide early which paper to sit — a decision that is much easier to revisit now than after the form closes.