BigN8 is spot on. There are two vent stacks on the roof. The one for the lower toilet is on the northwest side, and gets the full blast of wind coming in from the fields. The other one is on the southwest side, which is not only lower, but is also sheltered by the back of the house and the peak of the roof, and by a mature Spruce tree that breaks the force of the wind.
I think the NW one also doesn't have a great baffling on it, something to work on when the weather gets warmer.