I think that the verbal and reading comprehension are generally "easier" than bio, chem and the quantitative section (obviously not for those whose first language was not english).
Therefore, even if we do fairly well on the section, since everyone does fairly well we will have a lower percentile score. For example, if we answer 80% of the questions correctly in the reading comprehension and the quantitative sections, our percentiles would be a lot higher in the math vs the reading. (did that make sense).
This doesn't explain really, really low scores, but I think it explains why there is not a ton of extremely high ones.