If it shipped 2 million copies in the first month and then managed to sell enough to merit further shipments bumping it up to the 2.6m it was at, then it sold through those copies fairly quickly or retailers wouldn't have ordered anymore.
I don't know why people misinterpret shipped figures so much.
Look at my post again. I'm not misinterpreting anything, they sent out a lot of copies, and they sold a good chunk of it too, but it still took awhile to clear through all the copies floating around. Not to mention the game was multiplatform, which accounted for the increase in copies. Regardless, the game didn't seem to have much legs after that. It was front loaded.