What am I missing about the AMT?

The one-year "hold harmless" patch to the AMT that House has approved (Senate concurrence expected momentarily) boosts the exemption for a married couple from $45,000 to $62,550 for the 2006 tax year only. That creates the certainty that we get to enjoy a rerun of this debate a year from now, which I know we all will greet with joyous anticipation.

But given that the change takes effect only in 2006, why is it scored as a $12 billion revenue loss in 2006 and another $18 billion revenue loss in 2007? I thought these calculations were done on a tax year basis--even if the added AMT isn't paid until 2007, shouldn't it be credited to 2006, when the liability is incurred? Alternatively, if they are really accounting for cash flow and not liability, why isn't the whole cost in 2007? No one can know until the tax year is over whether any AMT will be due or not.

One additional issue: given that the cut in the tax rate on capital gains and dividends was followed by an increase in tax revenue from these categories, why is the extension of that rate scored as losing revenue? Why isn't the past prologue? I realize that the revenue increase is partly because economic growth was better than expected, and that the exact relationship between that growth and the lowered tax rates can't be quantified with precision (the old static v. dynamic scoring debate). Still, given the actual experience I think that the extension should be scored as, at worst, a neutral on revenue.