This is due to the formulation of phosphors used during the construction of the tube. See how color temperature relates to the "warm white" and "cool white" labels? All well and good for your office or home, but not nearly good enough for our corals and inverts! Hence we rate our tubes by K, and that other thing known as wattage. K stands for Kelvin, or the color temperature, another confusing term for CRI. As a general rule, aquarium related fluorescents in regard to reef habitats should not fall below the 5500K rating.
Please keep in mind that different companies use differing terminology for the same thing. CRI, K, Spectrum, Color Temperature, etc., all refer to the type of light their products produce. Actually, there are sound reasons for using differing names or labels, copyrighting being amongst the most common. Just like car companies use "4X4" or Quad Drive, it still means the same thing!
Basically, and without getting into a deep science seminar, the lower the Kelvin, the more yellow the light. Hence, the higher the Kelvin, our light appears starkly white or blue-white. Getting way up there into the 20,000K range, the bulbs actually appear to glow dark blue! Shades of the blacklight craze! Oh no, don't EVEN go there! Blacklight has tons of that nasty ultraviolet, and our animals don't like that at all! Save it for the bowling alleys or fuzzy posters!