We actually know CBO only scores what they are told to score meaning their projections will never be accurate. Here is a good example:
"Overpayments are a rampant problem in the unemployment insurance system. The federal government and states overpaid an estimated $14 billion in benefits in fiscal 2011, or roughly 11% of all the jobless benefits paid out, according to reports from the U.S. Labor Department."
If actual subsidies are projected to be $681 billion what will the real world amount be when you add in fraud, gaming the system, and government incompetence?
At 11% that is an extra $75 Billion.