Pfizer (PFE) says its proposed merger, which it announced on Monday, will take its tax rate down from 23.5% to between 17% to 18% by the first full year after closing, which will likely be in 2017. Based on Pfizer’s math, the drug company will save about $1.2 billion in taxes that year. Not bad. (Allergan, because it is already based in Ireland, won’t see much of a difference in its tax bill.) But at least one expert thinks Pfizer’s actual savings in the first year after its combination with Allergan (AGN) could be nearly triple that, or $3.3 billion a year.

Robert Willens, an independent tax expert, says it has to do with the way Pfizer already shifts around its income. In 2014, for instance, Pfizer reported that it had $3.1 billion in overall tax expenses. But while that was Pfizer’s reported tax expense, that’s not what Pfizer actually paid in taxes. Earlier this month, The Wall Street Journal noted that much of what Pfizer reports as tax costs, roughly two-thirds, it doesn’t actually pay.

That’s because a significant portion of the taxes that Pfizer reports are on earnings the company has generated overseas. And Pfizer already claims that it makes all of its profits and then some overseas. Last year, Pfizer said it made nearly $17 billion overseas and lost nearly $5 billion in the U.S.

Pfizer, like all U.S. companies, doesn’t have to pay U.S. taxes on foreign earnings until it brings that money back to the States, which it seldom does. Last year, for instance, Pfizer had $2.2 billion in deferred U.S. income taxes, money it owed in taxes on foreign profits that it didn’t actually pay. Nevertheless, Pfizer books that as a tax expense because, as a U.S. company, it has long said that it would eventually have to.

But once Pfizer is an Irish company, after the Allergan deal goes through, it won’t owe U.S. taxes on overseas earnings. Subtract that $2.2 billion from the $3.1 billion Pfizer said it paid last year in taxes, and you get a tax bill of just $900 million. Compare that to the $12 billion in pre-tax profits that Pfizer had in 2014, and that’s how Willens gets to an expected tax rate of 7.7% after it is domiciled in Ireland. Based on that tax rate, Pfizer, which is expected to earn $21 billion in pretax profits in 2017, could see its tax bill drop by $3.3 billion the year after the Allergan deal is completed.

“The rate that Pfizer is reporting it will pay seems like it doesn’t represent hardly any of the benefits it will actually get from moving to Ireland,” says Willens. “It’s not unheard of for companies to report a higher tax rate when the deal is announced so as not to rub salt in anyone’s wounds.”

Pfizer declined to comment on how it calculated its estimated tax rate after the deal.

It’s a little trickier to calculate what kind of hit U.S. taxpayers are going to take from Pfizer-Allergan deal. Remember, even though Pfizer was booking those expenses, it wasn’t actually paying U.S. taxes on its foreign income. Pfizer will continue to pay taxes on its U.S. income, even after it moves to Ireland, which was $948 million last year. So the U.S. government will not see a drop in tax revenue because of the deal.

Still, the assumption, at least from Pfizer’s accountants, was that the company would eventually have to pay those taxes on foreign income. Once it moves to Ireland, it won’t have to. Pfizer has $21 billion in deferred taxes that it booked but did not pay. That’s all money in back taxes that U.S. taxpayers will lose out on, not to mention the billions Pfizer won’t pay in the future.

Last year, the Congressional Budget Office estimated that inversion deals would cost the U.S. Treasury $19.5 billion in lost tax revenue over the next decade. The Pfizer deal alone suggests the number will be much bigger than that.