It’s not so much the length of the words that makes them get excluded, it’s that they’re generally articles or prepositions that don’t add much to the meaning of the term. Just think about the words that won’t get capitalized in a movie or book title.
Beyond that, if you’re looking for “rules”, you might be taking it too seriously. People tweak initialisms & acronyms around to make them memorable and roll off the tongue – something that’s far more important than making them “proper”.
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