Don't believe this data at face value. Improving your MCAT score is far more attainable than what these data suggest. This report from AAMC has been around for a long time and they like it because it makes them feel that their testing instrument is "so good, you're score will effectually never change." This is the classic example of data only being meaningful if you consider all of the variables/context. I've helped many, many students attempt a second or third time and I have NEVER had someone improve by less than 4 points, the average is probably 10-15 if you do everything I ask you to do.
These data are misleading because A) they simply sum up all of the examinees who retake, and because B) most students who retake the exam do very little different the second time compared to the first. When I say "most students," I mean that I've hardly encountered a student who truly "metamorphosized" their approach after getting a poor score. Often, they retake a short 2-3 months later, with very little additional preparation. Usually, they do the same low-yield things they did last time. At best, they took a lecture course from X Company the first time, so they try a lecture course from Y COMPANY the second time. Big deal. To increase your score significantly on a retake, you must CHANGE the things that matter, and change them in a BIG way. You must study WAY MORE. You must dramatically increase the CONCEPTUAL nature of how you study, dramatically increase the accuracy of your practice or the depth of your practice exam review. Input = Output. If you put in the same things you get the same results, which is basically what these data say because MOST students put in essentially the same input on a second attempt. BUT, if you drastically change the input you WILL GET a different output! I promise...