Canon’s “brightness” histogram is best treated as an in-camera exposure guide, not a precisely documented physical brightness metric.

Based on the community answer, the key point is that camera histograms are derived from the camera’s JPEG rendering and processing settings, not directly from the raw sensor data. That means picture style, contrast, tone curve, white balance, and other JPEG decisions can affect the histogram.

Also, many cameras flag clipping conservatively: a luminance/highlight warning may appear when any individual color channel is near or at clipping, not only when the final pixel is pure white. Some cameras may even treat channels as clipped slightly before full 255. This can make the camera’s “brightness” histogram differ from a histogram you compute later from exported data.

So the practical answer is: Canon’s brightness histogram is manufacturer-defined and model-dependent, based on the processed JPEG preview, and may not exactly match standard luminance formulas like BT.601 or BT.709. It’s useful as a general reference for exposure, but not as a strict, reproducible scientific measure of scene brightness.