They are forecasts and there is always an element of uncertainty. I can give you a very simple example to illustrate the uncertainty.
If the price of a barrel of oil drops to $10 and the economy continues to do well, the price of gasoline will fall and the price on carbon will not have much impact. People will drive more and have less fuel-efficient vehicles because they can afford it more. Factors of that type can have a significant impact on forecasts.