All LED's really have different characteristics, even LED's from the same reel, even from the same piece of crystal. They can be binned, which does better, but you will still have variations.
If you carefully measure the point at which each individual LED lights up to the same level (place it next to a "standard" LED that is lit by a power supply and a series resistor set to -- do not use a battery, as the voltage may change more than you want over the course of your testing).
The LED which illuminates last will have a higher forward voltage when it reaches the illumination of your standard.
Think the water analogy... The LED's that light up are like sprinklers with their orifices at a lower height, so as you turn the water spigot slowly on, the water finds the easiest path, and comes out of the lower sprinklers. The one up on the hill gets nothing until you increase turn the spigot on even more, and the pressure "reaches" the lone sprinkler up on the hill, where it finally starts emitting water.