Each June, health insurers send the government detailed cost forecasts for providing prescription-drug benefits to more than 40 million people on Medicare.

No one expects the estimates to be spot on. After all, it is a tall order to predict the exact drug spending for the following year of the thousands of members in each plan.

However, year after year, most of those estimates have turned out to be wrong in the particular way that, thanks to Medicare’s arcane payment rules, results in more revenue for the health insurers, a Wall Street Journal investigation has found. As a consequence, the insurers kept $9.1 billion more in taxpayer funds than they would have had their estimates been accurate from 2006 to 2015, according to Medicare data obtained by the Journal.

Those payments have largely been hidden from view since Medicare’s prescription-drug program was launched more than a decade ago, and are an example of how the secrecy of the $3.5 trillion U.S. health-care system promotes and obscures higher spending.

Medicare’s prescription-drug benefit, called Part D, was designed to help hold down drug costs by having insurers manage the coverage efficiently. Instead, Part D spending has accelerated faster than all other components of Medicare in recent years, rising 49% from $62.9 billion in 2010 to $93.8 billion in 2017. Medicare experts say the program’s design is contributing to that increase. Total spending for Part D from 2006 to 2015 was about $652 billion.