It's not so much as the LED-64 sensing the amount of voltage being applied to the LED, but the amount of current going through the LED (to control the brightness). You can think of it as the LED-64 internally setting a resistor to a value in order to achieve the desired current so that you do not need to add an additional one.
The Forward Voltage of an LED corresponds to the amount of voltage that needs to be applied in order for the LED to turn on. You can use any voltage to power an LED as long as it is higher than that voltage and as long as you choose a resistor that limits the current through the LED below its maximum rated current.
The min, typical and max Forward Voltage ratings that are listed in datasheets mean that the Vf can vary from LED to LED. On average, a random LED with that part number will have a Vf of around 3.3V, but some LEDs can have a Vf as high as 3.6V. This means that when you have an LED attached through a 220 Ohm resistor, the same LED part numbers can have some slight variations in brightness.