In fact, it is precisely the JPEG ISO value that all the manufacturers publish.
They do so because JPEG (or any RGB) output is the visible output that photographers use.
So when you select ISO 800 on your camera, you’ll have a JPEG ISO at 800,
but the RAW ISO will be at (for instance) 550.
The JPEG results are achieved by playing with the tone curve shape.
This is absolutely legitimate: the ISO standard allows manufacturers to use this JPEG value.
They are not cheating.

ヘッドルームあな！とかどこに書いてるんだ？穴