The rules have changed a few times over the years. The two question system is the current preboard determination.

I've heard them ask additional information when the person does not say yes to one of the questions, but starts telling (in detail) why they think they need preboard, but it still doesn't answer one of the two questions.

A recent example was someone in an arm cast who said no they didn't need a certain seat and no they didn't need assistance, but went on to say they can't have that arm in an aisle seat. The GA said "so you do need a specific seat", and the passenger then was able to see what a certain seat meant.