There is not really any good 'spin' on why one would get a low %-tile on the RC section. But there probably is some great anecdotal humor that could be used.
Bio/Chem/Quant measure different thinking abilities then say Verbal/RC. The former requires good route memorization and knowing which equations to use (for plugging and chugging) in each situation; the latter requires using context clues, current vocabulary depth (which for most scientists like us are mostly in the realm of scientific terminology), and basically being able to critically read articles (e.g. decipher) what someone is trying to explain.
That being said, there are numerous individuals who are stellar on Bio/Chem/Quant (in the 90%+ area) yet score less then 25% on the Verbal/RC, which could be attributed to English as a second or often third language. English is definitely a tricky language compared other languages.
People may disagree with me on this, but I feel Reading Comprehension abilities can be greatly developed if you read news magazines (Time, USNEWs, Economist, etc...) , scientific journals/articles (Nature, Cell, Science, etc...) , other analytical papers, and possibly even Harry Potter rather then reading academic text books solely.