The first set of four images is taken outside on a bright day without the use of a tripod. While I have tried to keep the scene the same there is some movement which would change the histogram. Also while the scene remains the same there is slight movement from the sunbathers plus also the pedalo is moving.
(click on image to enlarge)
The second set is the same scene but using a tripod. While the histogram moves less the movement again of the people in the scene does cause this to move.
The third set of pictures is indoors where the lighting should remain static and the camera is handheld. Again the histogram moves probably because the camera has moved slightly for each picture.
The fourth set of pictures is indoors but tripod mounted. Ignoring the fact the picture is not quite straight the histogram still moves but is less. Maybe some of this is that the camera moved just slightly when the shutter is depressed.
Sets five and six are handheld and tripod mounted respectively. This time there is less in the scene and therefore the variations between each picture in the respective set and across all the images should be far less. This is the case but even on the tripod mounted there is still a variation.
What is interesting is the sensitivity between what the eye sees and what is registered on the histogram. Looking at some of them where there is just a slight difference to my eye they look the same. Will investigate further at some point as to how much the histogram has to change before the eye can see this change.