I did a google search and came up with the following:
As per Wikipedia: In the video game industry, AAA (pronounced "triple A") or Triple-A is a classification term used for games with the highest development budgets and levels of promotion. A title considered to be AAA is therefore expected to be a high quality game or to be among the year's bestsellers.
Sure, it's Wikipedia, but all of the following results said the same thing in slightly different words. So there is a perception of expected sales or quality, because people conflate big budget and high production values. Doesn't mean the end product is going to be good. We see overhyped big budget games fail frequently. However, there is a perception and expectation that quality and sales come from putting a lot of resources into the development.
So I agree that not all AAA games are great. However, I disagree that this changes the definition of the phrase. The phrase is about budget and promotion. Quality is assumed but not necessarily realized. Can't change that, just because there are heavily promoted games with huge budgets that end up sucking.