Hemingway wrote regarding his iceberg theory:
If a writer of prose knows enough of what he is writing about he may omit things that he knows and the reader, if the writer is writing truly enough, will have a feeling of those things as strongly as though the writer had stated them.
In "The Killers" many things have been omitted. Hemingway bragged that he had left more out of that story than any other story he had written before. He said he left out the entire city of Chicago.
Among the things Hemingway left out are:
What did Ole Andresen do to make someone want to have him killed?
How did they know he was living in Summit?
Were they planning to kill George, Nick, and Sam after killing Ole?
Why didn't Ole show up at six o'clock?
Why didn't he seem frightened when Nick told him about the killers coming to the restaurant?
If George knew the clock was twenty minutes fast, why didn't he reset it?
There are many other unanswered questions. Evidently Hemingway himself knew the answers to all of them and decided not to give the answers in his exposition or dialogue because of his so-called iceberg theory. By leaving out explanatory material he could focus on action and dialogue, making his story more dramatic.