So they say, "will authorize sale of X lands" and then all sorts of administrative reasons happen that no sales actually occur. The bill supposedly already has a clause that at least some of the tax breaks will expire in 4 years. So when the CBO and others project the increase in the national debt it looks not too bad assuming that actually increase in taxes happens. When it doesn't happen, and if (heaven forbid) the Repugs are still in power in 4 years the actual debt net add basically doubles. The CBO has no choice other than to compute based on what the bill currently says, even though common sense and history says that being reality 4 or 10 years from now is highly unlikely.