I believe the passenger front airbag on/off is set by weight of the occupant, with 100lbs being the threshold. The idea is that if someone is too light (e.g. a child), the airbag will do more harm to them than if the airbag did not deploy. Thus, it turns the airbag off.
I've had the same situation happen to me when I had someone about 100lbs in the passenger seat. Some days it would be off, and others it would be on. I think it may just be best to let the car do it's thing, as I have a hard time believing they set the 100lb threshold arbitrarily.