Well for one thing , kinect doesn't see things in 3-D no more than another other web cam. The claim Kinect "sees" in 3D is a little misleading as most people think 3D as like how they see. Kinect cam sees a completely flat image. There are ways to try tracking something 3d on a flat image . For example Move uses the size of the ball on the image to determine where the move controller is in Z-axis. The more pixels the ball image uses the closer the ball is relative to the camera. Kinect tracking the Z axis how the strength of IR light bouncing off an object.
I can think of two major disadvantages of Kinect tracking you sitting vs standing up.
1) You are closer to surrounding objects including what you can sitting on. (standing puts distance between you and other objects) While our brain can do this with ease even with one eye close yet it's a lot tougher for and computer to do this.
2) Your images itself takes up less pixels on the camera which means Kinect is recieving less information.
So while Kinect may work sitting down I doubt it will be able to track as well as standing up. This is good reason Microsoft wanted developers to develop games at least at launch standing up.
