Bad Math on the Weekend Train

I am on the train home from a long, full day in the city. Lots of walking, lots of sightseeing, great times with great people.  I have two ladies and two girls with me. The girls wanted to sit on the upper level of the train on the way home - no problem! We found some seats together. The two girls are facing each other, the two ladies are facing each other, and I am sitting behind the two pairs.  

A few stops in, a man got on the train and made his way to the upper level. He stopped near us and looked back and forth. Then he said, "Excuse me ladies, can you turn one of those seats around so I can have a seat?" We looked at each other. We looked at the seats. We looked at him. We looked at each other again. My adept sister looked right at him and said, "there's a seat right back there." As he found another seat, we all looked at each other again and made sure that our math was correct: if five people are sitting in five seats and one of us got up so he could sit down, there will still be one person without a seat. That's just good math.  His math was bad...very bad.