What are the major differences between a normal war story and this story, and why is Tim O'Brien telling it this way? I notice he talks a lot about...

O'Brien's story is different than other traditional war stories.  The story can be viewed as a piece of creative nonfiction.  Most war stories are either fiction or nonfiction.  However, O'Brien,  maintains that the truth is not so easily set down on paper.  Whose truth is expressed in the story?  Is it Cross's?  O'Brien's?  That is not easily revealed.  That is part of the point:  the experience of war and its repercussions are not easily explained.  Many true war stories try to explain what happened.  O'Brien doesn't do that.  He simply allows the reader to experience all of the confusion, fear, and exhaustion the soldiers felt.  

Note how O'Brien does not tell the story in the traditional fashion.  It does not have a chronological beginning and end. Instead, key events, like Lavender's death, resurface.  This implies how his death haunts the men.  It is always at the edge of their minds, and it cannot be laid to rest easily.  If the story were told in a traditional format, Lavender's death would likely be the climax of the story.  All of the events would build up to that.  The event would change all the soldiers and carting Lavender's body off in a helicopter would be the resolution.  However, O'Brien has structured the story so that it is not as 'neat' as a traditional war story.  The form embodies the confusion and stress of the soldiers themselves.