I'm saying photons here, because there are so many words that have very specific meanings that are not typically understood (Nits, Candela, Lumens, Luminance, Brightness, Illuminance, Luminous Flux (Lux), and Luminous intensity) by the lay person. However, just looking at the 5500K black body the power density of any Blue is well below that of any Green or Red, and since the energy per photon is higher at shorter wavelengths (blue) that means the photon flux is significantly lower (about half the number of photons/sec/m2 of green).
The reason that Cyan (between Blue&Green) and Yellow (between Red&Green) subpixels added to the full pixel can be more efficient for white (or unsaturated colors) is that they are detected better by the eye (perceived brightness) for an amount of power or photons than a primary Blue or Red.
