I tend to agree with you, but to some extent certainly luck plays a factor, it is impossible to be perfectly prepared. I got a 34R (12/11/R/11), which is in the 92.0 - 94.3 percentile on that test, and it is possible that I got 35 questions wrong. Certainly, I could have been on the cusp either way in every section and been 3 questions away from a 31 or 3 questions away from a 37, that is a pretty big range that reasonably a good deal of luck. There was only one question I had to totally guess on, but if that was a hard question on cell bio, which i knew well, I could have gotten a 35 maybe.
Furthermore, I will agree that verbal is nearly no luck, but PS and BS have some luck effect. Lets say I take Gen Chem/Gen Bio Year 1 of college, and Physics/Orgo Year 2 of college, which is common at my school, then many at my school take the MCAT after sophomore year so they have time to retake if needed (this is what I did). If you study, no matter how hard, you will not be 100% at both. Let's say you get 90% and physics and orgo and 80% at gen chem and gen bio. You get 9 passages on BS and PS, if you get 5 of orgo/physics and 4 of gen chem/gen bio you "know" 85.6% of the questions, if you get the other way, which is sheer luck, its a 50-50 chance, you know 84.4%. That difference may seem minuscule, but on AAMC 9R (whose curve I used as reference for this) that is the difference between an 11 and a 12 in both PS and BS.