Actually, it has to do with the job sectors where women and girls who drop out find themselves. For example, those women work in hair salons, spas and any home-care jobs, as well as beneficiary attendants. Those sectors are very poorly paid. By comparison, boys who drop out can get a vocational diploma in construction, mechanics and welding.
So those two job sectors are really underpaid. That is mainly why they earn less. It is also because they have children and often work part time.