Hi Manni, look @ page 44 of the PDF, it says how MaxCLL is calculated.
setmaxRGB= max(R,G,B) , so the max is one of the R or G or B value.
For MaxCLL interpretation, the unit is equivalent to cd/m2 when the brightest pixel in the entire video stream has the chromaticity of the white point of the encoding system used to represent the video stream. Since the value of MaxCLL is computed with a max() mathematical
operator, it is possible that the true CIE Y Luminance value is less than the MaxCLL value. This situation may occur when there are very bright blue saturated pixels in the stream, which may dominate the max(R,G,B) calculation, but since the blue channel is an approximately 10%
contributor to the true CIE Y Luminance, the true CIE Y Luminance value of the example blue pixel would be only approximately 10% of the MaxCLL value.
So if the brightness sub-pixel is blue, its not really be as bright in nits as the MaxCLL number is showing.
Stacey Spears mentioned this @ 26:13 of the following video: