What do you mean "equally likely is the explanation"? You mean to say that you have no reason to believe the one over the other. On the other hand, I was under the impression that (I could be delusional), for the majority of universities, the average SAT/ACT scores of the students in the science/engineering/math departments is actually higher than the other departments. In my opinion, the better argument would have been that liberal arts majors (non science, math, engineering) are more numerous, which would explain why more of such students graduated with honors. Then your argument that those majors are just as difficult or more difficult would be shown to be plausible or implausible depending on whether it was true that both a larger proportion of LA majors earn honors compared to STEM, while at the same time being filled with "statistically less capable" students (SAT scores, high school GPAs). If these statements turned out to be true, then maybe your case still could be saved if you could show that on average, English (etc.) majors work harder, which is why they are over-represented. If this also turned out not to be the case, then your argument would seem less plausible than ever.
There are plausible arguments that could explain why non-STEM premeds would have better success with the MCAT even in the case that it were found that on the average, non-STEM majors in general are slightly less capable.
One such explanation could be the that there are so many science classes required to enter medical school, it is most natural to major in a STEM field. More classes apply to the STEM student's major. Therefore, it is likely that those who major in non-STEM might be more intellectually interested in their major. People who are intellectually interested may be smarter than those who aren't, on the average. E.g. these people who major in the non-stem fields might be the same people who have a stronger than average interest in their major (even compared to others in the same major). For the same reasons, those premeds who major in unconventional fields can be more generally motivated. Also, those premeds who major in unconventional fields are aware of the fact that they aren't necessarily looked down upon (and so feel free to major in them); many students feel they are expected to major in some sort of science, or feel pressure to do so. The fact that they know this may indicate that they are more savvy than the average premed; the (relatively speaking) premed who loves English but is a premed and so on that basis alone instead majors in Biology might be less bright in comparison to the premed who loves English but does indeed know any major is considered acceptable for admittance into medical school, and weighs the pros and cons of sacrificing all of those science credits which they could use in progress of another major, and decides they love Joyce and Shakespeare so much that they take the plunge.
Another plausible explanation is that those who major in non-STEM majors develop their reading ability to a greater degree. The fact that they still have to take all of the same premed classes and so aren't at a disadvantage in background knowledge (more advanced science classes rarely are significantly helpful, e.g. it seems rare that it would affect a persons' score by one point, given that the MCAT tests only basic [as opposed to advanced] factual information), and at the same time are exposed to more reading material means that they aren't disadvantaged in the sciences, but they are advantaged in the verbal fields. Furthermore, it is possible that have strong comprehension (probably strengthened in a book-heavy major) abilities .could actually help in the ubiquitous passage-based format of the test. So even though a person is not otherwise subject to a relatively (compared to STEM) difficult field, they are still exposed to the material required for the MCAT, and they are further advantaged by the fact that they now have augmented verbal abilities. This explanation is most relevant to majors who read many pages of material/prose. Reading many pages of science books is not the same because 1) it takes far longer to read a page of STEM material, and a lot of that time might be trying to decipher a mathematical expression or memorizing scientific facts and 2) science books don't use the same level of prose as those of non-STEM majors, for example the vocabulary doesn't generalize as it's so specific, but perhaps more importantly, they don't use a wide range of syntactical structures. The fact that you are reading at such a slow rate and in a way not useful for reading passages, especially MCAT verbal, supports the fact that reading STEM textbooks is a different experience than reading prose. Less relevantly, I heard that your comprehension abilities are improved by a far greater amount by the act of understanding, as compared to the act of attempting to understand something very dense. Ignore this last sentence if it tempts you to ignore the rest of what I wrote.
Those are my thoughts.
