When does one equal 64? When there’s a whiff of funny business going on.

Or more than a whiff. In December, China reported that imports from Hong Kong increased 64.5% year on year. For the same month, Hong Kong reported that exports to China increased 0.9%. Why the huge gap involving two sides of a trade that should roughly match? Chalk it up to suspected capital outflows across the restricted border, say economists.

Despite stepped-up efforts by China in recent months to prevent capital from leaving its shores, a flood of funds continues to head overseas in search of better returns, with trade flows the latest front in efforts by investors to circumvent restrictions.

“This is very consistent with the idea that people are using trade accounts to get money out of China,” said Cliff Tan, head of global markets research with Bank of Tokyo-Mitsubishi UFJ. “This is a very time-honored system in developing countries, you over-invoice imports and under-invoice exports to get money out of the country.”

China isn’t sitting by idly. In recent months, it’s stepped up prosecutions against illegal money changers, encouraged more companies to invest in China, enhanced supervision over cross-border transactions and blocked foreign banks from trading onshore and offshore currency, among other steps.

But the capital keeps flying away, or is spent by Beijing to intervene and keep the yuan stable. China’s foreign reserves have fallen by $663 billion since June of 2014, including a record $108 billion in December. That’s in spite of trade surpluses during the period averaging nearly $50 billion, which act to boost reserves. China has seen its foreign-exchange hoard decline to $3.3 trillion at the end of 2015 from a peak of $3.99 trillion in mid-2014.

“Even if China maintains tight controls, there are always loopholes,” said Mizuho Securities Asia Ltd. economist Jianguang Shen. “That’s what the Chinese government is worried about.”

Under strict capital controls imposed by Beijing, consumers are only permitted to purchase $50,000 worth of U.S. dollars each calendar year. But manipulated foreign trade deals offer a way around tightening restrictions, say economists.

In an example of how this is done, a Chinese company might import 100,000 widgets at $5 apiece from a Hong Kong partner or subsidiary company, paying them $500,000. It then exports the same widgets back to Hong Kong at $1 apiece, receiving $100,000 from the Hong Kong entity. The goods are back where they started, but $400,000 has now moved offshore.

ANZ Group economist Raymond Yeung said the very large spread between offshore and onshore yuan rates recently has encouraged the potential use of trade channels for financial arbitrage.

Economists said the best way for China to stem the flow is to calm markets, take control and reduce expectations that the yuan is going to decline sharply in value. “They need to quite clearly intervene and not allow the market to set the exchange rate,” said Mizuho’s Mr. Shen, who previously worked at the European Central Bank. “And clear communication is needed. They haven’t been very transparent lately.”

According to China’s General Administration of Customs, China imported 1.05 trillion yuan ($164.1 billion) worth of goods from Hong Kong in December, a 64.5% increase. On the other side of the border, Hong Kong’s Census and Statistics Department reports that Hong Kong exported HK$168.13 billion ($21.57 billion) to China in December, up less than 1%.

“It’s Mr. and Mrs. Chen sending money out,” said Bank of Tokyo-Mitsubishi UFJ's Mr. Tan. “You need to calm their fears. And in order to do that, you need to stabilize the economy.”

--Mark Magnier and Chester Yung. Follow Mark on Twitter @markmagnier and Chester @chester_yung.