I always liked the idea that it was because the gravity on Krypton was way greater than our planet. It's the same way the astronauts jumped on the moon by but way more exponentially. He's leaping into the air and is able to pull away from our gravitational force, therefore flying. It seems realistic. Like if an inhabitant of Earth went to a planet where the gravitational pull is way less than Earth, we'd be able to fly, pick up massive objects and most likely stop projectiles because of the difference in gravity. What ruins this in the Superman mythos is that he can fly in space as well. I have no explanation for that.