I stand corrected.
I don't believe that its for more natural growth,
from the ADA australia website, I read this as it provides the best Veiwing of the plants, i.e. it provides the green light to make the plants prettier to the human eye.
also most plants do not come from a poetic forest floor area, most come from lakes and marsh areas in water less than 2m deep, so the effect of water absorbing the light is very minimal. if a plant come from these areas they are normally sciophytic and grow in areas of the tank that should be shaded from above by other plants, naturally filtering and reflecting the light to a more green wavelength anyway.
from the Barr report
or you could just look at what light chlorophyl absorbs, as proven many times over by science....