Was your Prestige account converted from a Chairman card? It seems more likely that AA is an internal code referring to the discounted annual fee paid by former Chairman cardholders. (My card was converted from Chairman MasterCard, and I do see the "AA" code.)
Mia, my card was not formerly a Chairman card. However, I did upgrade it from a Citi Thankyou Premier account last year. If we are to surmise that it is an internal code for a discount, perhaps it has to do with the recent increase in annual fee from $400 to $450. Since most people on this thread so far have reported having the card for a while perhaps those who applied since the fee increase to $450 do not have the AA code on their account?