November 2011 was a huge month sales-wise, so it's not surprising that last month's sales were slightly smaller. Blops 2, AC3 and Halo 4 were no match for MW3, Arkham City, Skyrim, Uncharted 3, Skyward Sword and the Nov sales of BF3.
Next year the yearly comparisons will become easier. The first half of 2012 contained only 2 blockbuster releases: Mass Effect 3 in March and Diablo 3 in May. GTA V alone will easily sell more than those two lesser-known games combined, and there are so many BIG games coming in H1 2013 - including Bioshock Infinite and The Last of Us - that surely the NPD figures will finally start showing growth