Choosing Between 'A' and 'An'

Choosing between the indefinite articles "a" and "an" is pretty easy, right? Then how do you explain how often I see stuff like "They visited a AAA-aproved resort"?

[audio:http://www.grammarunderground.com/wp-content/uploads/2012/01/0077-Indefinite-Articles-A-vs-An.mp3|titles=0077 Indefinite Articles A vs An]

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