Inside a led
A "Natural" or "Characteristic" voltage develops across a LED when it is correctly connected in a circuit with a current limiting resistor to allow a current of between 1mA and 20mA. This voltage is shown in the table above and we normally use the lower value for each color.
However the table shows the voltage varies quite a lot and this depends on the actual crystalline construction of the crystal and the way it is manufactured.
You cannot change this and that's why you need to measure the voltage across the LED when building some of the circuits.