Here is what the people making those devices are trying to tell you.
- the If vs. Vfwd curve comes the led datasheet.
- The thick green curve comes from those nice folks at Atmel. They are the I-V curve of a mcu's pin.
The two curves together determines the output current on the led, without any resistors. It is about 15ma If and 3v Vfwd. That's due to the pin's internal resistance (0.3v / 15ma = ??? ohm).
If the pin had no output resistance, its I-V curve would be like that red line at 3.3v. The output current would then be at 30ma.
That is essentially the basis why some designs have no serial resistors.
