We have a screwy language.
American English is no longer a language with strict meaning in it's vocabulary - not that it ever really was, I suppose. It's more a collection of formal Queen's English at it's base, heavily blended with very broad colloquial terms, auxiliary slang, and contextual you-figure-it-outs, augmented nowdays with successively more techno-speak and ethnic spice. Plus, I've read that we have more compound words than any other language. I've also been told by several folks who speak numerous languages (between 5 and 9) that American English is by far the hardest Western language to master because of this lack of rigidity and its free-form style.
Anyone who watches the show NCIS sees this regularly with Ziva, the Israeli team member who can never get a colloquialism right.
In other words Alan, yeah we have a screwy language. For all intents and purposes, semi-annual and bi-annual are the same - only different . . .