In The Boy in The Striped Pajamas, in the last chapter (on page 163), how is what is actually happening different from the way that Bruno sees it?

The Boy in the Striped Pajamasis a fictitious novel set during the Second World War. Bruno and his family move from Berlin to "Out-With," Bruno's mispronunciation of Auschwitz, where his father will be the commandant. When the family arrive in this "desolate place," Bruno can hardly believe that he has had to leave his friends and a very "fine" banister for sliding down, behind. He is not allowed to explore and has no chance...

The Boy in the Striped Pajamas is a fictitious novel set during the Second World War. Bruno and his family move from Berlin to "Out-With," Bruno's mispronunciation of Auschwitz, where his father will be the commandant. When the family arrive in this "desolate place," Bruno can hardly believe that he has had to leave his friends and a very "fine" banister for sliding down, behind. He is not allowed to explore and has no chance of making any friends. When he decides to defy his parents' instructions and explore along the fence where "those people" are described by his father as "not people at all" (chapter 5), Bruno meets Shmuel and, ironically the boys find that they have a lot in common, despite their vastly different circumstances.

The boys meet regularly along the fence and Bruno remains fascinated by all the people on Shmuel's side, even feeling jealous because, despite Shmuel telling him how unpleasant it is, Bruno does not really believe him, and longs for friends to play with. Towards the end of the novel, Bruno and his mother and sister will be returning to Berlin so Bruno and Shmuel have to say goodbye. Shmuel has some pajamas for Bruno, as he promised, so that Bruno can see what it is like before he returns to Berlin, and also to help Shmuel to find his "Papa" who is missing. 

Bruno is shocked by what he sees in the camp and when everyone starts "marching" Bruno and Shmuel are caught up in the middle. The rain comes down hard and Bruno becomes anxious to go home, telling Shmuel that he will catch a cold if he stays. The boys have no idea what awaits them and Bruno believes that they are taking shelter from the rain. He is content to wait for it to ease off. This is very different than the reality which awaits Bruno and Shmuel as moments later the boys are dead as the group which was ushered into the "airtight" room was the latest group of people being "marched" to its death.