Perhaps the ratings are related to expectations rather than a series of fixed points grading the business. My last trip on Air Asia was absolutely fine and they even gave out umbrellas when we disembarked as it was starting to rain and it was a short walk to the terminal. I was perfectly happy with this airline; I expected little other than the trip and got that at a cheap price.
My last BA flight was in J to VRN. The Gatwick lounge was fine, the meal on board was a bizarre curry, the VRN lounge awful and featured an exploding coffee machine and there was a compulsory salad meal served during the return flight. Did it meet my expectations? Not if I had studied the description of the J experience that BA pedals!