Color Temperature

[Ray]

The fact is that the 6500° bulb's output spectrum most closely matches that of sunlight and the blue backscatter of the sky - what the plant would see in nature. Adding wattage in the redder end is unlikely to "force" better flowering, but cannot hurt.

Just speculation on my part, but back before phosphor technology advanced to where it is today, it was very common to mix "warm" and "cool" bulbs in order to try to approach a decent spectrum, as either one alone was not great, and I'm guessing that thinking has carried over, even though it's no longer necessary.