Your explanation has 3 major problems:
1st: It makes no sense.
2nd: It attempts to rationalize why bio majors have lower acceptance rates to med school than humanities majors.
3rd: It makes no sense.
No, it's not quite that "simple." Where is the data showing that the premeds majoring in humanities only decided to go into medicine halfway through their college career? Where is the data showing that all the bio majors came into college as premeds and knew they wanted to be doctors since age 7? From what you're saying, the humanities majors just switched tracks from careers in academia, law, business, diplomacy, or **insert anything else here** to medicine once they got the high MCAT score...if they didn't earn the high MCAT score...they would've just taken their 4.0 humanities GPA and applied to grad school or law school.
I have the data about the average MCAT scores among the different majors and the evidence is damning. Bio majors: be prepared to feel embarrassed. Humanities majors TIED bio majors on the BS section of the MCAT and score substantially higher on the VR section (as expected). If I remember correctly, I think the humanities majors even had a higher average PS score than the bio majors.
I think the reason that humanities majors have a higher acceptance rate to med school is because they DON'T fall into the "cookie-cutter" trap that most bio majors are happy to place themselves in. Out of the 30,000+ med school applicants in one year, over 20,000+ of those were bio majors. This means all of those students had the same classes on their transcripts and I'm guessing most of those students probably also did the same cookie-cutter, biobot extra-curriculars (premed society, hospital volunteering, lab research, etc).
While none of these extra-curriculars are "bad" and certainly will not reflect negatively on the student...doing these ECs will make it excessively difficult to DIFFERENTIATE onseself from the legions of other premeds applying in the same app cycle. I'm guessing it was much easier for the humanities majors to differentiate themselves from the legions of other premeds because their extra-curricular interests were more varied, they had classes on their transcript that most adcom members aren't used to seeing (think "Philosophy of Friedrich Nietzche" vs. Microbiology). The key here is not NOT be biobotic. I know a philosophy major who is applying to med school this cycle and he works for two non-profit organizations, is helping a professor write a book that will be published in about two years, and spent a semester abroad teaching English to children in the Phillipines. I think his list of ECs (and the ones I mentioned are just the highlights) will help separate himself from the legions of people applying with hospital volunteering, lab research, etc.
Well, those are my opinions. I'll be back to post the links to the average MCAT scores by major...I can't right now because I'm at an internet cafe.