The forward voltage drop across LEDs varies - they're not all just 2v. One thing that influences Vfwd is the colour of the LED. Basically the shorter the wavelength, the larger the voltage drop. A red LED is about 1.6v, blue may be over 3v (though even "identical" LEDs of the same colour may vary, particularly if they're from different batches). The forward voltage drops of the LEDs in the circuit are real ones, taken from LEDs I have in my parts box (I think one of those blue ones thinks it's a UV LED). If you're using multiple LEDs, especially of different colours, you're best to use separate limiting resistors.
