The term "Vf" means forward voltage. That is, the amount of voltage the LED needs in order to draw a certain amount of current - 20 milliamps is a very typical value you'd see with most round 5mm LEDs.
With a higher Vf, the LED would draw less current than the other LEDs in the same group using the same power supply. And while the other LEDs in the group may be being overdriven to the point of degrading and changing color, the one with the higher Vf would be receiving less current, and would be less likely to degrade in that manner.