Histogram appears wrong - too low

The histogram appears to be reading low. I have clouds that blow out under a certain exposure setting, but the histogram shows nothing above about halfway on the x-axis. I expect to see data on the right wall of the histogram at the same time I see clipping in the image.

If I'm misunderstanding how the histogram data is scaled relative to what is displayed in 8 bits monitor, that should be explained, and ideally indicated with reticles on the histogram.