Fact: Someone with a spouse/partner and/or minor children is going to have more responsibilities, some of which cannot be rescheduled like school, or farmed out like a child too sick for school or daycare.
Fact: Single people have responsibilities too and have responsibilities that can't be rescheduled. And while they have less responsibilities, they also have no help in fulfilling those responsibilities, it's 100% on them.
Fact: Children need time with parents and spouses/partners need time together to form strong family ties and mentally healthy individuals.
Fact: Forming strong family bonds and being a healthy individual does not stop at the age of 18.
Fact: Multiple people families take more time to meet all obligations.
Fact: Scheduled blocks of time matter as much as quantity. Most businesses aren't 24/7 and neither is sunlight.
Single people should not be punished for being single by means of preferential treatment given to people with families during scheduling. People with families need time with their families and scheduling specific blocks of time is the only feasible way of managing that.
So bottom line, everyone should get equally sucky schedules, unless someone has a need for a certain time that is so strong a need it outweighs other people's needs.
Maybe that's what's going on. Maybe single SETL has something serious scheduled in the afternoon, a burden heavy enough specific scheduling is needed.