How do sequence numbers work?

When I look at my boarding pass for a flight tomorrow I see I am "Sequence number" 157. Now, I thought that was based on the order in which one checks in (if I'm able to check in T-30 hrs, for example with a connecting flight I often get Sequence number 1-5), however, for tomorrow's flight, I'm on a 737-800 with 154 seats, and I'm checked in at T-22hrs...

.....am I supposed to believe that 156 people have checked in for that flight before me? (i.e. A total of 3 over capacity, 22hrs in advance?!). Or is there something I'm missing in my belief here?

Curiosity more than anything else.