This might also explain why Clark was able to hold his own against two Kryptonians at once and snap Zod's neck, something that shouldn't be possible with their powerset. Clark grew up on Earth from a very young age (it looks like only a few days old), while Zod and his companions were only exposed to our Sun and atmosphere for a few hours. Zod getting the ability to fly so quick may be explained by his nature as a warrior (which, IIRC the dialog during that scene basically states). So because Clark grew up on Earth he is truly invincible while the other Kryptonians are only incredibly strong. Jor-El also stated something about how Earth's environment would make Clark's skin and bones infinitely stronger, which also helps explain why Clark was able to snap Zod's neck. Zod's bones were still very weak comparatively.