Hemingway believed in showing only what was going on with the fewest and simplest words possible to do it. He doesn't explain why his characters make the choices they do. He simply presents the scene and leaves the rest to the reader to work out. Thus, only 1/8th of his meaning (so to speak) is on the surface. The rest must be surmised, based on the few clues he provides and (most importantly) upon the reader's understanding of human nature and history.
I've long thought that Hemingway wrote novels the way playwrights write plays: The dialogue and basic setting is provided, but most of the underlying motivations--attitudes and emotions--must be determined by the actor. When you read a play, you must determine whether a character is good or bad and how they'd say their lines, and why; the same is true of reading Hemingway.