There are many reasons why they may be different.
- Many cameras will show luminance histogram clipping if any one of the color channels is clipped.
- Many cameras will show a color channel as clipped at a level of around 245-250 (i.e. early).
- Some camera’s histograms just lie pretty badly (my D800 did)…
- The camera’s histogram is based on the camera processed jpeg and is highly unlikely to match the raw file data (if recording/editing raw).
The histogram in LR is almost certainly more accurate than the one in your camera.
And interestingly, LR’s histogram may not match the camera’s histogram for a jpeg image either (reasons 1-3 above).
Here is a D850 jpeg image review on the camera.
It shows all three channels and the combined luminance as touching the right side (clipping) and the highlight warning shows a large area on the upper chest as clipped for all four (R/G/B/L).
This is the same jpeg w/o any edits in LR.
The histogram doesn’t show any luminance clipping (short of right edge), and if I hold alt while selecting the exposure slider it shows me that the small clipping warnings are for the blue channel only.
And the raw file histogram looks somewhat different as well (as expected).
You can do some things so that the camera histogram(s) and image review more closely matches the raw file histogram(s); at the expense of less usable jpegs SOOC. I use the “neutral” camera profile with reduced contrast (minimum) and brightness (-1) settings so the image review is closer to a raw image in LR with the same “neutral” profile applied. You can also go down the path of using uni-wb in camera; for totally useless jpegs, a lot of hassle, and a bit better histogram match.
But none of that is absolutely necessary… all you really need is to develop a good idea of the difference between what your camera typically shows vs the same raw/jpeg file opened with defaults (and it’s highlight recoverability).
Many who shoot ETTR record raw files; push the in-camera histogram to where it is just showing clipping (slightly climbing right side; at least touching), and rely on the ~1stop of recovery capability. But you have to experiment with your camera to know how far you can push it. Also, do not ETTR by increasing the ISO… there’s no point to doing that (ISO is not light/exposure/data).