Time spent studying definitely impacts the range of grades one can expect on an exam, albeit to a limited extent. Look at it in extremes -- the guy who spends 6 months studying torts is going to do better than the guy who started studying Monday. Nobody disputes this. And yet if this is true, the same logic applies to the projected grade ranges for the student who studied 6 weeks before exams and the student who pulled 4 all nighters the week before -- intelligence and other external factors aside, the former will always do better.
Sure there is an arbitrary element to law school grading, but it's NOT a complete crapshoot. The people who say this are the medians who get an occasional A and an occasional B minus -- the assertion supports their experience, but by no means reflects the truth. The more time one spends going over practice exams and model answers the better one prepares one's self to spot issues and re-create arguments. HTH