I think that most 40+ are luck in the sense that the score is not reproducible (unless one NEVER makes dumb mistakes); those people would likely score high thirties (36-39) on a retake. I don't think that people who have "32" potential luck into a "40," rather of the
many people who have "40+" potential, only some will pull it off and most will score high 30's (36-39). Many times, the people with the lower score (for 1-2 pt. differences)
missed fewer questions (if their score is more balanced).
Let's consider a balanced 39 (13,13,13) versus a balanced 42 (14,14,14).
The person with the 39 probably missed 6 (maybe 7-8) questions:
50/52 PS (maybe 49)
38/40 VR
50/52 BS (maybe 49)
The person with the 42 probably missed 3 questions:
51/52 PS
39/40 VR
51/52 BS
What seems a little messed up is how a person could score something like 12, 15, 15 (still a 42) but they could miss ~ the same number of questions (around 6-8) as the person with the balanced 39:
46/52 PS (maybe 44-45)
40/40 VR
52/52 BS
Hence, it appears easier (in that you can miss more questions overall) to get a 40+ if you are lucky enough to pull a 15 (vs. 14 or 13) on one section AND a 13-14 on another, b/c then you can miss a bunch of questions to get an 11-12 on the third section.
* note that I say
lucky enough not because people who get 15's don't deserve them, but b/c MANY people who have the potential to score one or more 15's on the MCAT make 1-2 dumb mistakes and end up scoring in the 13-14 region.
These numbers are based on the number of questions missed to get a certain subscore on the average AAMC practice test.
edit: I don't know the exact number, but you can miss an obscene number of questions (I think ~13) to get a 40 if you get 10, 15, 15:
39/52 PS (or BS)
40/40 VR
52/52 BS (or PS)
However... It would take a lot of luck AND smarts to pull two 15's. I mean, you might also miss ~13 questions but get a 12, 12, 12 for a balanced 36 on most administrations (great, but not a 40):
48/52 PS
36/40 VR
47/52 BS
I got the "missing 13" estimate from the following thread, where some dude went through 7 AAMC tests and figured out how many Q's you could miss (on avg.) for a 10:
http://forums.studentdoctor.net/archive/index.php/t-395740.html