Two numbers in the same title sounds redundant.
Call of Duty 6 Modern Warfare 2?
Bleh.
The case has Call of Duty on it, but not a 6. WaW was the 5th one in the series but it's not CoD5. Therefore, MW2 is not CoD6, since there technically wasn't a 5.
What's weird though is, even though the case has Call of Duty clearly labeled on it, when you put it in it only says Modern Warfare 2. It seems they had trouble making up their mind.