Because LED lamp lifetimes are significantly longer than some of the competing products in the baseline (incandescent, halogen, and CFLs), a baseline lamp replacement model is used to determine how the baseline changes over the measure life.  Slides 8-17 of this presentation give some context as to how this baseline replacement model was developed and what it entails. In the model, the RTF assumes that lamps that reach the end of their lifetime are replaced probabilistically with the market mix of competing technologies.  Savings vary through the measure life because of this changing baseline.