There are two factors. First, the reduction in tax rate has a cost to it, but again when the economy is growing, these costs will be more than offset by the growing economy. That's why you end up with still higher revenue despite the fact you reduced the taxation rate. It all depends on how much you reduce the taxation rate, but for the tax reduction we had, it was enough to be more than offset by a growing economy.
Now there's the other issue that it's very difficult to estimate, to get something very precise about it, but there's also this issue and this view among economists that when you reduce tax rates it leads people to work more if you reduce the tax on working, it leads businesses to invest more if you reduce tax on investment, and so on. There's the feedback normally into the economy over time when you reduce tax rates. You incent people to save more, invest more, or work more. There's potentially some of these second-round effects in these results. Some economists like to think that these things are there.