How are the ratings calculated?
Each of the class ratings is calculated based on the rate at which you kill, die, and/or gib. Special class capabilities are taken into consideration, including reviving, giving ammo or health, reparing, and destroying objectives. Each of these items is weighted differently for each class. Playing a supporting role as medic, for example, is favored over one who plays medic simply for the health bonuses.
Each of these ratings is scaled to a value between 0 and 100. Thus, it is possible to have a class rating above 0 even if you have not played that class. This translates to a player(s) having played a class and obtaining a negative rating before scaling. The ratings are not absolute and depend on how well you are doing in comparison to other players for that class.
The four class ratings are then weighted based on the time you have played each class and used to form the overall rating.
http://stats.ecgnetwork.com/rtcwstats/faq.php