This is largely because Obama assumes the passage of legislation that the CBO doesn't, and he assumes those laws will generate far more revenue over the next decade. In 2024, the spending/revenue gap (i.e., the annual deficit) in Obama’s budget amounts to 1.6 percent of GDP. CBO’s projected deficit is more than twice that, at 4 percent of GDP.
The difference lies largely on the revenue side – Obama assumes hearty revenue increases coming from the elimination of tax breaks benefiting the wealthy.
Of course, these revenue increases assume a compliant Congress that works with the White House to pass major new legislation in the coming years. And the likelihood of that particular scenario is almost zero.
More on the budget: