LED Vf's will vary batch to batch, ESPECIALLY if they are from different bins.
This is one MAJOR reason that LED's need to be driven from a constant current supply. The Vf obviously also drops as the temperature increases. In addition, there is an 'aging' effect, typically the Vf will drop over the first few hundred hours of use.
So, you have Vf variations within a bin and more extreme between bins. You have Vf variations based on junction temps. You have Vf variations due to aging. Enough variables for you to play with?
cheers,
george.