The "AAA" terminology comes from the finance and investment world. AAA refers to creditworthiness for bonds, i.e., how safe it is to invest in it. When suits from publishers were talking about AAA games at E3 in the 90s back when it was a networking and business conference, they were talking about how sure fire a hit they thought the game would be. The more confident they were in the project, the more they would invest in it (or try to convince others to invest in it, depending on the situation). Somewhere a long the way, game journalists heard the phrase and started throwing it around in their publications to the point that it lost its original meaning.