NVIDIA nTune Question: Overclocking

Hey all,
So I just bought a new 8800 GTS 640MB OC and I am very happy with it overall. Just for fun I thought I would try to overclock it a bit more, 625 core 900 memory from the factory overclock of 575 core 850 memory. The overclock itself worked fine and the card is not running over 70 C according to nTune, but I did notice something odd. When not in a game the core clock reverts to the factory overclocked settings, however the memory clock stays as it should according to my overclock settings. I thought it was weird that only the core clock reverted to the old settings. I can see that the core is at my settings during a game by quickly alt-tabbing out of the game and it will stay at 625 until the next updated reading from nTune, at which the core will drop back to 575, but the memory stays the same throughout. I have only ever owned ATI and I know that sometimes control center software changes for 2D/3D settings. In this the case with the NVIDIA software nTune, that it is simply changing setting according to 2D/3D clocks? Also, is there any way to turn this off?

Thanks