group of people sweeping the floor and cleaning
Some janitors are sweeping floors and cleaning the campus late at night.
Not all people sweeping the floor are janitors. The floor might not be on a campus. The people might not be cleaning at night.
Just becayse a group of people are sweeping and cleaning does not mean they're janitors.
Just because there is a group of people cleaning, it does not mean they are necessarily janitors. They also aren't necessarily on a campus or that it's late at night.