I was in aperture priority and had the same f stop, same ISO, same focal length and same lighting conditions and my camera, a D90 used 3 different shutter speeds ... underexposing, correct and overexposing a few times. I had center metering chosen. Can anyone explain why this would happen, almost sounds like I hit the bracketing button (I know my fingers did hit it once ... I still say too many buttons ) but I don't think it happened more than once. Any suggestions?