kirk -
Probably because for the past 3-4 months, the X360 has been outpacing last year's 3-4 months. October, for example, the X360 via NPD was up 40%+ vs. last year. A significant increase.
When you have a system that's been selling alot better than last year for a decent bit of time (since August), it shows that the trend is, the system will sell better.
The X360 has faced brutal competition the past few months - continued Wii dominance, PS3 price drops, all in the middle of RROD problems, Moore quiting, and such. Yet despite that, it sells oh-so well since August. That shows that even from analysis standpoint it'll do a bit better. Add in the huge holiday lineup that MS has, all culminating in December, and it'll sell great (atleast 1.4m).