I believe the AAA term actually originates from investing. In investing, a "AAA" investment is one where everyone is pretty confident that it'll be a positive return. It got a bit of use in the games industry to mean games that were expected to sell well no matter the what. It eventually got warped into just meaning big games with big budgets, and people started using the "AA" term to mean "like AAA but not as much"

The wikipedia article does a better job of explaining it

I'd bet on JRPG rank rules; D, C, B, A, A+, A++, S, S+ and as many S' they bloody want in order to make S tier worthless. Instead of doing the sane thing and adjusting the criteria for a rank.

But in this case there is no Ds Cs or Bs, only As.