Originally posted by twilhelm is it possible this is what the histogram is in context with, as opposed to the actual recorded image?
My thoughts are similar (see above) and follow the logic that since the actual recorded image does not yet exist, the screen emulation of that image might be the next best option. Just for kicks, check out the histogram with the cross-process custom image setting.
Originally posted by Mikesul But that does not make it very useful.
It is useful in the sense that a simulation is what is available. (Live view differs from the EVF or rear display on a mirrorless in that the lens is not typically stopped down for metering, framing, and focus on a dSLR.) A histogram of the metered sectors might be even less useful, particularly for spot or center-weighted where most of the scene will not contribute. I am not much for basing exposure on histogram details or extremes after seeing the differences between in-camera, Affinity Photo, and Lightroom for identical JPEG. If the target is a gray card and the histogram is centered, the exposure is nominally "correct" based on the incident light. If using TTL matrix metering and either extreme of an otherwise balanced histogram is clipped, the button that should be pushed is to consider a bracketed exposure with intent to merge as HDR. ETTR sounds good* unless the shadows are clipped to save the highlights. (I generally use an actual test shot histogram to make these decisions.)
Steve
* To be honest, Expose To The Left (ETTL) is probably the appropriate term for the practice of preserving highlights by intentional underexposure with intent of attempting salvage of the shadows in post.