What kind of metering and focusing, and do you know where the focus point was in each shot? Was the focus point locked, or was it left to move around? If you used single point focus, the focus point from image to image moved quite a bit, based on the framing difference of each image. Since the focus point chosen in each image, if you were in single point focus, hugely biases the meter setting by its different placement in each image, that could account for the first image's difference.
If you want to really see if you have a problem, putting the camera in a physically stationary position, such as on a tripod, is a must.