"Sometimes Kaplan administers very difficult ones to try to scare kids into buying a prep class."
I am not sure where you got that source, because I took a diagnostic Kaplan exam and they gave me exam #21 (I think one in 1997). They gave out an actual exam and gave us a score based on how the students really would have received if they took the exam in 1997.
there is no hard LSAT exam, because it's all curved in the end. i guess you can say the exam is "hard" if you are not good at RC for example, but that's irrelevant, because that's personal strength and weakness.