Why does bruno have a hard time understading gretels explanation for the fence?

Gretel, Bruno's sister, does not know what to make of this fence at first. Both children are disappointed and mystified by their new surroundings. Gertrude, however, is quick to catch onto the fact that children and families live beyond the fence. Gretel is easily indoctrinated to the Nazi idea that these are not really people. They are Jewish and "different from us". Bruno never really understands or, subsequently, agrees with his sister's new outlook.