This should be at the store in a few days and I am really quite excited for it.
I'm seeing 4/5 stars and 8/10 when they sound more like 6/10 or 3/5.
The thing is that their reviews aren't written well enough to express that they are disappointed with how the game compares to its predecessors, but on the whole is still good enough to take a score of 4 out of 5 or an 8 out of 10.
The reason some of these reviews seem so negative is because they are comparing the product to the ideal, which is why the reader would think the reviewer would give it a score of something like 5 out of 10, when the games are still better than actual games that fall into that bracket.
A lot of reviewers have a different process of writing reviews and deciding scores. When deciding on a score, you basically compare the game to something of a similar genre, which you or your publication (if you wish to main consistency with the website/magazine you are writing for) reviewed.
For example, say someone was really disappointed with Far Cry 2, and wrote a mostly negative review... they could still feel that the game was better than other shooters they have given scores in the 70 percentile, even if they were quite disappointed.
It actually comes down to the skill of the writer. He has to be able to express why he is disappointed, but what still makes the game worth a score of 4/5 or 80%
One final thing. The majority of the writers who I have noticed disappointed with FF13, have still gone on to state that it is still one of the best offerings on the Xbox 360/PS3.