One of the reasons for this is because NVIDIA doesn't want the GeForce 8800 GT to eat up too many sales of the more expensive GeForce 8800 GTS. Additionally, it also gives NVIDIA some more headroom in case the ATI Radeon HD 2950 PRO comes too close for comfort.
The GeForce 8800 GT will be launched on October 29th and according to NVIDIA it will score about 10769 in 3DMark06 but it's not clear on which system the card was benched.