BeagleBone Black LED current/brightness
Small signal LEDs are typically fraction of a watt, with a target 20 mA forward current at the recommended forward voltage. This specific led has a VF of 2.8 ~ 3.8 Volts at 20 mA (specifics depend on the bin, and ultimately on the led).

This spec is for the recommended, 100% continuously on brightness for the estimated life of the led. These specs are for Illumination designs, not simple Indication.

That LED is used as a simple indicator light. In this design, the goal is not to light up as much as possible, its just to show a status.

At very low currents, less than 3 mA, the relative brightness of the LED is so low compared to the brightness at 20 mA, that its considered insignificant. This is a subjective spec. But the actual, objective brightness of the led at less than 3 mA is still visible. It's just not as visible as 5mA, or 10mA or 20mA. At this point, it's no longer used for illumination.