Completely and totally random question

This is an English question that has been bugging me for some time. I know the rule of thumb is that if the word begins with a vowel, then you use "an". However, if the word begins with a consonant, you use "a". Hence, you get, "an apple" or "a grape".

Now, there are two words I can think of off the top of my head that do not follow this rule. Hour is certainly one since, although "h" is a consonant, it is also silent so we say "an hour". I get that one. The one I do not get is USB. I saw it written the other day as "a USB" and realized that that is how it's said but I'm not sure why. Is it because it is an acronym? Does anyone know the answer or hazard a guess?