They could just be cautious. Microsoft has slowed down production in Jan-March before and suffered from some shortages. My guess is that they aren't intending to book as much production ahead of time and thus be more conservative. If the Xbox 360 sales continue to do well up to and beyond November when Kinect launches its pretty obvious Microsoft will book some more wafers from the fabs.
Theres no point in their building up too much inventory and there still is the question on whether they will shrink again to 32nm next year which would mean that unsold 45nm Xbox 360 S chips will be a PITA they don't want to manage.