LED manufacturers don't have enough control over the process to produce LEDs with a consistent efficiency or color. So after they manufacture them, they sort them according to color and the amount of light output for a given current. Conceptually, they have a number of "bins" they put the LEDs into according to color and output.
As their processes improve, they're able to produce a larger fraction of LEDs with better efficiency, so the availability of LEDs from higher efficiency "bins" increases.
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