The internet is a haven of explanations which seem plausible but came about by common mis
Think about it. Bimonthly, biannually, bisect.... none of those are ever used in reference to a situation of two occurrences/unit. They refer to one occurrence/two units.
Anyone who uses the term biweekly when they mean twice a week is a clueless as people who say "I could care less" instead of "I couldn't care less." Both may be in common usage but one is right while for the other it is merely understood what the person meant. They're not the same thing.