My wife the English teacher tells me it's all about connotations, or the emotional linkages we have to words that may be in addition to their "official" denotation.
In North America, "abnormal" makes people think of craziness, mental disease, and so on. On some sub-conscious level, a North American might say to himself "well, the engine is on fire, but I'm not abnormal, and my co-pilot probably isn't abnormal, so I'll skip that page in the emergency procedures".
My wife the English teacher also tells me that, in English, if a word appears more than once in publicly distributed documents, it is "official". Dictionaries record things after the fact and are usually several years, or decades, out of date. This is unlike some other languages (French comes to mind), where there are official dictionaries, enforceable by law.