Those figures are approximate for an LED (not white or blue) - one would get the information from the LED's datasheet. Every LED's datahseet will have a current/forward voltage pair, it is the most basic specification for the LED. White and blue LED's are a different type of semiconductor material so have a much different voltage (more around 3.5V to 4V)
Even from the datasheet the numbers are an approximation though, based on the mean values that they have tested their LEDs at. That is because the manufacturing process cannot produce LEDs perfectly the same all the time.