It’s not that I didn’t know that… It’s just that we don’t think about the rules behind our language. The interesting point was that none of my coworkers could come up with another exception to using “a” in front of a vowel, even though, after it’s pointed out, it’s exceptionally obvious…

Like I said, I’m just re-examining our language from a different perspective since I’m trying to learn another one.

Words that start with consonants use an if they sound like they start with vowels, like honorable or STS.

Words that start with vowels use a if they sound like they start with consonants. The “u” words you listed all start with a long “u” sound, which sounds like it starts with a “y” serving as a consonant, as in you. Words that start with short “u” sounds use an, since it’s a vowel sound — an unusual plan versus a usual plan; an ugly wound, etc.

“O” can also take a, for example, when starting a word sounding like a “w” — a one-hit wonder.