My theory is that Kaplan purposely makes its exams or its scaling system more difficult so that when you take the real MCAT you'll end up with a higher score than they "predicted," and therefore be happy that you paid the ridiculous amount of money it costs to take the class. This happened to me and most of my friends who took Kaplan. Of course, everyone's different, but I think the general trend is there...Kaplan tests are definitely tougher, so if you're scoring a 31 then you're probably on track to do well on the real thing.
I think I took 6 full length exams before my MCAT, and I never scored higher than a 31. I actually started doing worse right before the MCAT, getting 28s and 29s. But when I took the real thing I ended up with a 36R! I was ecstatic! My friends had similar jumps in their scores. I also think this is Kaplan's way of making you want to spread the word about how great their course is, and basically give them free advertising via recommending it to your friends.