Don't forget the histogram is based on the picture style set in camera.
It doesn't seem to make a huge difference if shooting raw, but it can mean that the raw file doesn't always reflect the camera histogram.
Magic Lantern has raw based histograms that aren't affected by the JPG data/picture styles at all
The histogram becomes based entirely on the raw data.
Exposure is a detailed subject. It's no wonder that people have problems.

