Companies of all kinds try to present their financial results in the best light possible to attract investors. One segment of the drug industry is bucking that trend, using an accounting method that narrows its profit margins.

The three big middlemen for prescription drugs, known as pharmacy benefit managers, or PBMs, had operating-profit margins last year of 4 percent to 7 percent, a level that lags the 16 percent average among S&P 500 companies. Were they to tally their revenue the way many middlemen in other industries do, their margins would more than double.

Several drug industry analysts said that booking revenue in a way that shows lower margins might have helped the companies deflect criticism of their pricing practices. “It hides a lot. It’s as simple as that,” said Ravi Mehrotra, a partner at the MTS Health Partners investment bank.