https://phys.org/news/2019-01-women-men-equally-effective-wage-labor.html

"Some attribute the pay gap to perceived gender differences in wage contract negotiations," says Holona Ochs, associate professor of political science at Lehigh University, "or, to a belief that women undermine their own bargaining position by extending too much trust to others in negotiations."

In other words, some perceive men as more effective negotiators than women and assign this as a factor to explain the wage gap, in whole or in part.