@dyslexia,
What is strange about it?
You take the forecasted sales w/o the government program, you then add in the increased sales above what you forecasted to get the net impact of the cash for clunkers program.
For example, there is a forecast for the month of November. If the government creates a program, you measure it's effectiveness by seeing how much better November comes in over/under your forecast.
You cannot attribute every car sold during that time to C4C. If there were no C4C, cars would have still been sold, no?