The Intensity Scale (sometimes called the Gray Scale) not only controls the contrast within all displayed images but it also controls how the Red, Green and Blue primary colors mix to produce all of the on-screen colors. The steeper the Intensity Scale the greater the image contrast and the higher the saturation of displayed color mixtures. So if the Intensity Scale doesn't follow the Standard that is used in all consumer content then the colors and intensities will be wrong everywhere in all images.
In order to deliver accurate color and image contrast a display must closely match the Standard Intensity Scale. Figure 3 shows the measured Intensity Scales for the iPhones alongside the industry standard Gamma of 2.2, which is the straight black line.
Note that the Intensity Scales are very similar and overlap on the graph.