Science Fiction? Mythology?
Quick: name the last science fiction book to win a Pulitzer. Or a National Book Award. Or a Man Booker Prize.
Can't do it, can you? Science fiction is usually considered genre writing—like, when you go to a bookstore, there's a separate section for it, just like fantasy or romance. And genre writing doesn't get much respect. Ray Bradbury did a lot to change that, even though his writing doesn't sound all that much like science fiction.
Ray Bradbury has long been known as a science fiction writer, and the stories take place on Mars and involve lots of science fiction-y things, like telepathy, Martians, and robots. So why do we say put a question mark after science fiction?
Because even though The Martian Chronicles uses a lot of sci-fi techniques—like being set in space and talking about rocket ships—Bradbury isn't really interested in science and the reality of Mars. Occasionally he reminds us that we're on Mars by pointing out that there are two moons or that the atmosphere is thin. But his Mars is very much a fantasyland version. What really interests Bradbury is the universal truths of people facing the unknown.
In other words, he's writing a version of mythology. His Martian Chronicles are origin stories to explain the way people behave when they're facing a new civilization or a grand new adventure. Even the word "chronicle" emphasizes the scope of the stories, since a "chronicle" is often used to refer to the history of a people.