Ezra Klein explains,
The CBO in estimating future spending looks at current spending in Afghanistan and Iraq and assumes that spending grows by the rate of inflation. So CBO estimates of future spending do not take into account plans to wind down both wars. However, the estimates used by Paul Ryan and Harry Reid assume both wars will be wound down as planned and count the money not spent as savings.
There’s some truth to this argument, as I’ll explain in a minute. But the GOP is trying to have it both ways. Boehner uses the Congressional Budget Office’s deficit estimates. He doesn’t subtract trillions because he doesn’t believe the agency’s war-spending estimates are faulty. Nor do I remember him calling the savings from Paul Ryan’s budget — which Boehner voted for — fake.But the Congressional Budget Office counts trillions in war spending in its budget baseline, and Ryan’s budget cut a trillion dollars from the Iraq and Afghanistan wars.
In fact, cutting war spending was one of Ryan’s largest sources of savings over the first decade. The following table, which you can find in larger form in this document (pdf) at the House Budget Committee’s Web site, estimates them at $1.04 trillion against the Congressional Budget Office’s baseline.