I understood the K rating to be an indicator of the colors of light in various bulbs. Ie, soft white has a K of 3700, meaning it contains mostly blue and green light which plants don't need much of. Full spectrum indicates that the light produced contains oranges, reds and some purples... these are the 'rays' or wavelengths most beneficially to plant growth. Sometimes bulbs labeled 'plant' bulbs are higher in these wavelengths, but to our eyes they look weird. So, full spectrum contains nice looking light to our eyes, and a good assortment of wavelengths to make the plants happy.
When I specified the K rating, I was simply stating that any bulb can say it is a 'daylight' bulb, but only the bulbs labeled 5000K at least, or in this case 6500 K are true full spectrum bulbs.
Bulbs with say 18,000K really are just pricey unnecessary bulbs. The plants don't 'get' anything more out of them than normal full spectrum bulbs.
Just to clarify things,
Cathy