Xbox 360 NPD and our attach ratios are vastly different: Explanations?

[Squilliam]

NPD TOP TEN:

1. 360 LEFT 4 DEAD    ELECTRONIC ARTS (07)
2. 360 CALL OF DUTY: WORLD AT WAR*    ACTIVISION BLIZZARD (O2)
3. 360 SKATE 2    ELECTRONIC ARTS (03)
4. 360 LORD OF THE RINGS: CONQUEST    ELECTRONIC ARTS (09)
5. 360 GEARS OF WAR 2*    MICROSOFT (04)
6. 360 FALLOUT 3*    BETHESDA SOFTWORKS (12)
7. 360 ROCK BAND 2*    MTV GAMES/ELECTRONIC ARTS (11)
8. 360 GUITAR HERO WORLD TOUR*    ACTIVISION BLIZZARD (05)
9. 360 NBA 2K9    TAKE 2 INTERACTIVE (25)
10. 360 FABLE II*    MICROSOFT (17)

Vgchartz positions in bold: