All reviews are written by a reviewer or from the point of view of a gamer that enjoys playing games in that particular genre. I mean, it doesn't make sense to have a reviewer write a review of a sports game if he can't stand playing sports on a console. While this may tend to bump up a game's rating a little, it is only through pure enthusiasm that this would naturally occur -- the very same enthusiasm that you would feel for a game in your favorite genre.
So, why do we use letters as grades? Because your teachers did! But seriously, we believe that letters, including pluses and minuses, will give you a better indication of what we believe the rating should be. If one game is rated at 92% and another at 97% the difference might seem a little ambiguous. On the other hand, we are sure that you will understand the difference between an A+ and an A -. If you still insist on pegging a percentage to a letter grade you can refer to this table.