k rating is VERY misleading when sold on bulbs, esspecially MH bulbs. the k rating is more of a marketing standard, than a scientific measurement to describe the color of the bulb.
10k = white
14/15k = white with a little blue
20k = blue
so obviously, each company will have a different interpretation of the k value, as 14k bulbs from different companies could be vastly different. and you also have to take into account the type of ballast which will greatly reflect the k rating.
for supplement bulbs, pure actinic, usually means a purple/indigo color around 420nm. for a true blue "actinic" bulb, you would want it around 450nm. even though 420nm to 450nm doesn't sound like much, its the true difference b/w purple and blue.
so basically, kelvin ratings are more of an estimate by a particular company. if a company claims an actinic bulb puts out a certain nano-meter color, it should be pretty darn close as thats a much more accurate measurement of color.