Every February, Forbes comes out with its widely publicizedForbes Billionaires list.
It's designed to put concrete numbers on something subjective and fluid: people's net worth. And Forbes says its ace team of reporters, and long experience and data on billionaires make it the best list of its kind.
"We've been doing this for over three decades," said Randall Lane, editor of Forbes magazine. "We have miles and miles of files, a lot expertise. We feel our numbers are as good as you're going to get."
As it happens, another billionaire list came out recently. It's produced by the Hurun Report, whose chairman, Rupert Hoogewerf, used to research rich lists for Forbes. The Hurun Global Rich List, like Forbes', gets broad publicity worldwide.
Hurun puts the world billionaire count at 1,867, with $6.9 trillion in wealth, while Forbes put it at 1,645 with $6.4 trillion in wealth—so they're in the same ballpark on global billionaire wealth.
But the two counters came up with wildly different results for China. Hurun said there are 358 billionaires there; Forbes puts the number at 152. That's a difference of more than 100 percent.
(Read more: China minting billionaires at 'phenomenal' rate)
Both lists define billionaires the same way: People in China with a total net worth of U.S. $1 billion. When it comes to ethnic Chinese living outside that country, Hurun said there are 457.
There are three possible explanations for these divergent billionaire counts.
First, you could say that either Forbes or Hurun is simply better at counting billionaires. Since both keep their exact methodologies and reporting confidential, it's hard to know which is superior. But each would probably argue that it has the more accurate numbers.
(Read more: Where the fleeing Chinese millionaires go ...)
The second reason could be more mercenary. Hurun's business is built on the Chinese rich and companies that sell to them. Its "Rich List" is sponsored by Chinese luxury property developer Star River Property. Hurun benefits as China's super-rich ranks grow along with the luxury companies seeking to enter the market.
As Hurun said in its own press release, "China's billionaires are shooting up the Hurun Global Rich List. This is why Hurun Report has set out on this quest to track down and rank the world's billionaires."
But there is a third reason that's more likely: Calculating people's wealth is often a guessing game.
Especially with owners of private companies overseas, there is little reliable data to go on, and valuations of those companies and assets can be subjective.
For instance, if you're trying to measure the exact net worth of a chemical plant owner in Shaoxing, do you also know the total debt, the true sale price of the business, or the family's other business connections and holdings? Maybe. But pinning precise numbers on such an opaque system is more art than science.
Divorce attorneys will tell you that many wives and husbands of wealth creators don't know their real net worth. The rich don't even know sometimes, as valuations can change so rapidly.
(Read more: World's richest now worth $6.4 trillion)
As British publishing tycoon and wealth philosopher Felix Dennis once wrote, "The richer you are and the more financial advisors you employ, the less likelihood there is that you can ever discover what you are really worth."
That's even more true for rich-list creators.
—By CNBC's Robert Frank. Follow him on Twitter @robtfrank.