I think camera autoexposure is sometimes too clever for its own good

If you want consistent exposures, shoot in manual mode!.

Autoexposure is great when the light is constantly changing, like when you’re moving from sun to shade, from outdoors to indoors – and indoor lighting can vary massively. In these conditions, autoexposure is a modern life-saver, as it will do automatically in a fraction of a second what it would take you precious moments to do manually.

So where’s the harm? Well, autoexposure is great in constantly changing light. But when the light is essentially the same for a whole sequence of shots, your camera is still adjusting the exposure each time.