Hi there,
The voltage is something that is a bit harder to control in the manufacturing
process. The way the device reacts to current however is roughly the same
all the time, or varies in a way that doesnt really hurt anything.
The small LEDs take 20ma but their efficiencies can vary, so one LED might
have a different voltage than the other.
This voltage is usually called the "Characteristic Voltage", but sometimes when
the average voltage among many devices is mentioned it is called the
"Nominal Voltage".
It's nice to know though that the intensity curve is drawn with Intensity vs Current,
so when we drive the LED with a certain current we know we can get a certain
intensity. We also, at the same time, know that the LED should be relatively
safe if we stick with the current recommended by the manufacturer, letting
the voltage go to whatever it needs to be for that particular LED.