This has been bugging me for the longest time. Call of Duty: World at War IS NOT Call of Duty 5, if it was, it would be called, Call of Duty 5: World at War.
People feel most comfortable slapping numbers on new games, when they aren't even direct sequels.
In this case, the game would be called Call of Duty 4: Modern Warfare 2.
It's completely nonsensical to increase both numbers by one, when it's the first sequel to Modern Warfare.