I'm sure you've all heard that Call of Duty sales have been on a decline, year after year. Analysts stated that the franchise was beginning to plateau. Well, they were wrong.
Although Black Ops 2 sales are particularly lower than that of Modern Warfare 3. Which in turn, was also less than the original Black Ops. Black Ops 2 managed to hit the $1 billion mark one day faster than Modern Warfare 3, which hit the mark in 16 days. So, Black Ops 2 sold less than Modern Warfare 3 but managed to raise $1 billion faster, go figure.
My guess is that Black Ops 2 is now on five different platforms as opposed to three. It's on the usual three, Playstation, Xbox and PC but it's also on the Wii U and the PS Vita with Black Ops Declassified. Throw in an insane marketing campaign, and it's totally possible.