In the Hardware Unboxed Podcast, Intel's Tom Peterson gave some explanation (@ 24:00) for the difference in transistor density:

1. There are dummy transistors included in the design for various manufacturing reasons, and there's no standard as to whether those are included in the numbers.

2. SRAM cells in particular are highly optimized and the amount of SRAM (e.g. cache memory) on the chip versus logic will have a big influence on the transistor density.

He said he's still asking around the engineering team for a more complete explanation but that's the supposition.