A Wall Street Journal investigation detailed how health insurers have regularly overestimated the cost of covering prescription drug benefits for Medicare enrollees, allowing the insurers to keep $9.1 billion more from 2006 to 2015 than if their projections had been accurate.

The Medicare Part D program requires private insurers to submit bids estimating how much it will cost them to provide benefits to patients, and then compares the plans’ bids each year to actual spending. CMS makes monthly payments to the plans based on these projections, and then compares actual spending to the projections at the end of the year.

But based on what the WSJ termed “Medicare’s arcane payment rules,” health insurers are allowed to keep some or all of the extra revenue when they overestimate the cost of providing the benefits. They are able to keep extra money up to 5 percent of their original guess, plus a portion of the money beyond that 5 percent based on a Medicare formula, the WSJ reported. If insurers underestimate the actual cost, they could lose up to 5 percent of their original estimate before Medicare begins partially offsetting further losses.

This has incentivized insurance companies, such as CVS Health, Humana and UnitedHealth Group, to systematically overestimate costs, based on trends uncovered by the investigation. According to CMS data obtained by the WSJ, insurers kept more than $1 billion in extra revenue every year from 2009 through 2013, including more than $1.4 billion in 2012. In the example of 2015, insurers overestimated costs by about $2.2 billion but only paid back $1.1 billion in taxpayer funds to the government.