I'd assume the JPEG can have some embedded colour profile. This profile specifies a transform between the JPEG's 24bit colour space into the monitor's 30bit col

I'd assume the JPEG can have some embedded colour profile. This profile specifies a transform between the JPEG's 24bit colour space into the monitor's 30bit colour space. For example, consider two JPEGs with embedded colour profiles. "Red" (255,0,0) in one image will map to some "Reddish" 30 bit value, while (255,0,0) in the other will map to a slightly different 30 bit value.