Well, there's your problem. Multiple, actually.
A 5 does - on most websites and in most magazines - not mean what you think it means. Most publications consider 6s and below games they do not recommend. Since they are many people out there who did enjoy Final Fantasy XIII I don't think a very low score is warranted.
Problem number two is that you're thinking that the avarage is constantly adjusted, making sure there is an equal amount of games on both sides of the middle line. It is not. This is not maths. This people putting numbers on things that more or less reflect how they felt about it.
Third problem is that AAA are not the only games out there. Why would it matter if most of them are above avarage or not? They are just one facet of the gaming industry. But don't forget these games have some stupidly huge budgets. It shouldn't be a surprise most of these games end up being atleast somewhat enjoyable.
Fourth problem is that what you are asking is not something that can be given. Some games generate a wide spectrum of opinions, others do not. You can't force people to think differently just to meet some imaginary quotum. If you want to make sure you read a wide array of opinions, use sites such as Metacritic wisely. Pick some reviews from the top, bottom and middle of the list and
read them. The texts often say more than the scores as grading scales can vary from site to site.
(Caption reads: winning.
Thank you, caption

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