Estimated budget surpluses are based on its doing in the current and future years what it promised to do in the past. This isn't likely. In recent years they outspent caps by declaring such predictable expenses as the census emergencies. This is an election year so the pressure to spend in tough congressional districts is increased. As a Washington Post story recently noted, the Republican leadership is busy giving embattled congressmen bundles of pork to take home and pass out on the campaign trail.
Further factors work to skew budget forecasts towards optimism.
If the law says a programs sunsets, the CBO doesn't include the money needed to keep it going. It's not their fault, but it understates the chunk of money needed to keep our government living in the manner to which it's become accustomed, since the only thing that sunsets in DC is, well, the sun. This has important policy implications, as we evaluate the proposed programs of the would-be presidents, viewing expensive promises with some skepticism and trepidation. Yet, policy history teaches that tax rates can be adjusted up or down but spending programs just keep on growing, as beneficiaries organize to work the system. And even Gore's tax cuts are more dangerous than Bush's, since they are mostly new social programs masquerading as tax cuts rather than straightforward cut in tax rates.