From the Wall Street Journal on one of the options to make the revenue numbers look better:
One possible outcome is that the corporate tax rate would start at 20% and rise in stair-step increments in later years. That move could shrink the size of the tax cut by at least $350 billion over a decade, making it about three-quarters of its previous size and potentially dampening some of the economic growth Republicans said they are trying to create.
Why cut the rate to 20 percent if you’re just going to ratchet the rate back up? More from the Journal:
If lawmakers choose to increase the corporate rate in later years, companies could deduct the cost of investments against a 20% rate and then face a higher rate on the profits those investments yield, the opposite of the incentive they would want.