When the tire shop checked the advertised diameter of the good year vs the firestone, they were the same, but actual roll out diameter of the good year was more. How can that be if the diameter is the same?

Rolling circumference is determined by the cord length of the tire, not the physical diameter of the tire. The only time rolling circumference = tire circumference is when the tread is wore down to the cords. What happens on a new tire is the tread flexs and slides against the surface when contacting the surface since the cords of the tire are much stiffer than the rubber tread; this is evident in tires that are roaded a lot by the rounding off of the leading edge of the lugs (leading edge is the edge that contacts the surface first).  The closer the physical circumference is to the rolling circumference the lower the wear (hence why new tires and smaller tires wear faster than older tires and larger tires).