plants can use a much wider spectrum than that. it's just most plants won't do it efficiently and those numbers are a mean temperature measured according to perception of the human eye. most bulbs k ratings are a mix of several colors that amount to an overall perceived color to the human eye, hence the use of the spectral graph, par meters and pur calculations. of course both 6500k and 10,000k bulbs both utilize light below and above their "temp ranges" respectively.
in retrospect it really just is that with 6500/6700k and 10,000k you're getting no brainer bulbs that will work well without having to test par output, monitor plant health and such due to manufacturers making good plant bulbs that produce those colors.
i have seen a 12,000k bulb that was tested at pretty high par levels. don't recall who tested it, where it was found or what bulb it was.
remember k ratings are according to human perception... uv is not visible to humans... plants can and do utilize uva and uvb for photosynthesis and other physiological activities. how much so would depend on species in question of course.
i have seen arguments that 2700k light is useless for plants. this is simply not true. not that it's likely to produce great results without a lot of power but it can and will be used if provided. that's about the color of an incandescent bulb. granted it won't penetrate well enough for most tanks and the power consumption/heat vs. light output is immensely wasteful.
