Josh Gordon, policy director for the Concord Coalition, said it has to do with fundamental approaches in how the two offices estimate the future. The CBO based its projections on the assumption that the Bush tax cuts would expire in 2010 and that a patch to fix the alternative minimum tax would expire, among other things. The White House does not; instead, it assumes that nothing changes in current law.

Gordon and another budget expert we spoke with, Brian Riedl of the conservative Heritage Foundation, said they believe the White House approach is more realistic because it assumes current policy will continue.