It is because there are not enough tickets at $126 for your group. The system gives you the same price for all your tickets, so it will use the cheapest fare bucket that has enough for all of you. You could buy in bunches - get three tickets at $126 and the fourth (or more) at $161. it would mean having to do separate check-ins, but that is doable. You might be able to link the reservations, but I'm not sure (but I'm sure that someone could tell you - lots of SW flyers here).