It isn't based on runtime. It is based on 1 amp (though this varies from light to light), not overdriving the LED, and heat. If output was based on runtime, we would have various lumen output levels, and the same exact run time on each light.
Let's use a 200 lumen HiCRI for example. The 219c 5700 is very efficient. At 200 lumens, you might be pulling only .8 amps. A 219b 4000 might be pulling the full amp as it isn't as efficient. There can big large swings in efficiency in the same bin of emitters as well. This is what is affecting run time. We base everything from a set lumen output, and use 1 amp as a base. We won't go over 1 amp draw.
You do have various output levels. 200, 300, 325...Anywho, since so many dreams were coming true, thought I’d throw more lumens out again.