It's really simple. This is from the Economic Policy Institute:

EPI adds:

Because there are many differences in the private- and public- sector workforces, the study controlled for several factors such as level of education (the public sector as a whole is more highly educated) and number of hours worked (public sector workers tend to work fewer hours), as well as the nature of the work itself. Many public sector jobs such as firefighter and corrections officer do not exist in the private sector, so the study compared workers of “similar human capital,” or labor market skills.