“We did the calculations and that is what it was, so that is what it is,” James Ueltschi said.

While the elder Ueltschi may have built a fortune of as much as $4 billion, according to data compiled by Bloomberg, much of the money wasn’t legally his at death, his son said.

‘Tax Planning’

“My father had various vehicles to provide for his various beneficiaries through tax planning that he did throughout his life,” Ueltschi said from the New York headquarters of HelpMeSee, a cataract-focused charity the family formed in 2010 to solve preventable blindness worldwide. “Those assets don’t belong to my father and they do not belong to his estate. The Giving Pledge is half whatever is left.”

According to tax returns, Al Ueltschi and his foundation have given $5.1 million to HelpMeSee.

How a fortune can be many billions of dollars and satisfy the Giving Pledge with what seems to be far less is a function of how death bequests are structured, said attorney Sloan. Typically, a percentage of after-tax assets is donated, rather than a fixed dollar amount.

“If you have $1 billion, you need to be sure that by the time you are done paying taxes, debts, heirs, etc., there’s $500 million to give,” he said.

IRS Settlements

In reality, a pledge to give half of a $1 billion fortune ends up being half of $600 million, assuming the standard 40 percent U.S. estate tax is levied. It can take years for the IRS to sort out in large estates in situations where a percentage is bequeathed.

The IRS declined to comment on estate tax procedures, agency spokesman Matthew Leas said in an e-mail.