I'm pretty sure you know the following but just in case. What you do not understand is because K (Kelvin) is temperature vs nm (nanomaters) are wavelengths. A certain Kelvin temperature consists of a nearly infinite number of wavelengths to produce the perceived color. Nanometers, on the other hand, measure a specific wavelength. So even if the Kelvin temperature appears amber, it's actually a combination of wavelengths at different nanometers. In other words, while a 14K or 20K diode will contain some of the wavelength measured at 420nm, it will also contain many other wavelengths. A 420nm diode should only put out light measured to 420nm.
I hope this makes some sense.