When the Obvious Hits You!

This is, quite possibly, the most random thing I have ever posted, but I just had to note this for posterity!

I was struck by the difference in how we write and how we speak. For instance, in the written word, we use the indefinite article "an" (rather than "a") before words that begin with a vowel. All words beginning with a consonant are usually preceeded by the indefinite article "a" rather than "an." However, in the spoken word, this precedent does not always feel natural.

For instance, I was typing out the phrase "a SBC history" (short for "Southern Baptist Convention"). Now, if I were to say "a Southern Baptist Convention history," there would be no problem. But should I attempt to say "a SBC history," I would end up tongue-tied. Saying "an SBC history" sounds much more natural. Why is this the case?

Because we do not pronounce the letter "S" with an a "s" sound! We actually pronounce the letter "S" with an "E" sound (short "e" + long "s", like "mess" or "best").

Weird...
Is that common knowledge or have I been blessed by the Grammar Muse? :-D

EDIT: My collegiate comrade, David Wickiser, advises me that written grammar really is based on sound. Thus, "an SBC history" really is the correct way to write it out, similar to "an hour" as opposed to "a hour." Thanks, David!