In a single year, Medicare reimbursed female physicians almost $19,000 less than male doctors, according to a new study published in Postgraduate Medical Journal.

The researchers didn’t think they’d see a gender gap when they analyzed more than 3 million Medicare reimbursement claims for 2012 for more than 245,000 providers. Still, they compared the reimbursements received by male and female physicians across 13 medical specialties to see if there was a gap. Even after adjusting for how hard physicians worked, how productive they were and their level of experience, a big gap remained against women doctors: $18,677. In almost every specialty, women were reimbursed less than men.

The largest gap by sex—$16,688—occurred in nephrology, and pulmonary medicine and internal medicine were the next worst for women being reimbursed. Specialties with the narrowest gap were hematology, medical oncology and critical care.

“We didn’t expect there to be such a difference across so many specialties,” says Dr. Tejas Desai, one of the study’s authors and a nephrologist at the U.S. Department of Veterans Affairs in Salisbury, North Carolina. “None of your factors, like gender or age, should matter into your reimbursement. It’s a menu: you do X, you get Y.” Yet in 11 of 13 specialties, the gap persisted.

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It isn’t the first time researchers have noticed a pay gap by gender in medicine. Recent research found that female doctors are paid about 8% less than male doctors, a difference that equals about $20,000. One explanation for that result may be that women tend to negotiate their salaries less aggressively than men.

But it’s not clear why gender would matter in Medicare reimbursements. Even Desai can’t explain it. “In this case there is no negotiation; Medicare sets it, and everybody has to accept it. We tried to make a study designed in a way so that the most common explanations are accounted for or completely eliminated from the equation.”

However, one factor that may be playing a role is geography, he says. If more female doctors live in parts of the country that are reimbursed less by Medicare, that could be contributing to the gender pay disparity.

Desai and his team are planning to account for those geographic differences in an upcoming study, which will track Medicare reimbursements over years and by specialty.

Write to Mandy Oaklander at mandy.oaklander@time.com.