Still unclear about shutter speed and aperture settings. Were they the same in all shots? You indicated they were all at ISO 100, but not the other parameters.
I think the fluorescent light comment has merit. If aperture, shutter speed and ISO don't change and the lighting doesn't change, then the exposure should remain consistent. But I could see where shooting around 1/50 or 1/60th could produce inconsistent results with fluorescent lighting. I would wonder if it could even throw off the metering.
I will add, however, that one reason I use manual mode is because I sometimes find the camera metering to be inconsistent in certain situations meaning I am not necessarily surprised at your experience.