Oh, those funny things

Why is it that my girls will stand around inside the house with their backpacks on for 20 minutes before the bus comes, but as soon as they get outside to wait for the final few minutes--plop! off they go?? I do not understand this. Is it some odd game of chicken? Also, why do they scream bloody murder ("THE BUS IS COMING, MOM! WE'RE GOING TO MISS THE BUS!!!") if I don't have them perfectly ready within five minutes of the bus arrival, but if they have to wait for more than two minutes outside, they will think of something random they want to take to school and come running back in? How do they suddenly forget that they could miss their bus? Makes me wonder. (And chuckle.)