Barred Rock are not easily sexed at hatch unless they have been bred for that. With different lines head spots are going to vary. A hatchery pullet often has a messier head spot than a well bred cockerel. The average seller of barred rock is not going to be able to guarantee that chicks are pullets. They probably have a good idea, but not enough to make guarantees about gender. If they can not guarantee, then they sell straight run.That's they point, why would you sell easily sexable birds as straight run?