If you are shooting under fluorescent lighting, you can use the same camera, the same lens, the same exact settings, etc. and still get varying results. That is because fluorescent lighting flickers.
As the alternating current powering the lights reverses polarity the intensity, color temperature, and spectrum of the light will change. In most countries the alternating current is running at either 50Hz or 60Hz which means the lights are flickering 50 or 60 times per second.
If you use a high enough shutter time you can even see the effect from one side of the frame to the other in the same shot. The shorter the shutter time the more noticeable the effect can be. It will still vary from one shot to the next depending on where the current supplying the light(s) is in its sine wave when the exposure is started. Shutter times more than about 1/2X the frequency of the electrical current powering the light will show this effect (i.e. 1/100 second for 50Hz, 1/125 second for 60Hz).
This is due to the way focal plane shutters expose the frame by moving a narrow slit across the sensor or film and the entire frame is not exposed at the same exact time. As you make the shutter time longer this reduces the effect. But frame to frame difference can be seen even at shutter times only a little shorter than the entire cycle of the lights' flicker (i.e. 1/60 second for 50Hz, 1/80 sec for 60Hz).