Then you're conflating two different transformations. Squashing 241,0,0 and 255,0,0 is the transformation from P3, as performed on that image. As I stated in

Then you're conflating two different transformations. Squashing 241,0,0 and 255,0,0 is the transformation from P3, as performed on that image. As I stated in the last bit of my comment, the transformation from sRGB to the display's native color space, as performed on images without a color profile, CSS colors, etc. [edit: and of course images tagged as sRGB!] is far more subtle.